Following the successful etymological investigation into the origin of the Robin Hood name and mythology – see the major investigation on this site -- the prospect of a similar resolution of the mystery of the roots of the other major English, nay British, legendary figure could not but beckon. As is universally agreed, there has been no satisfactory etymology in respect of King Arthur thus far. I had put off embarking on such a quest, assuming the luck I had with the Robin Hood project would not be repeated, given that no 'way in' had suggested itself: there was no clue handed to me, as had spurred me to look into the Robin Hood name (an encounter with elderly residents of Hood Hill in South Yorkshire, who proffered the tradition of a mythical association and meaning of the name of the hill). It was not until 2015, after some updating of the Robin Hood text, that finally I began as a diversion a little provisional digging. Amazingly, the pretty obviously right derivation presented itself within the first day of work. The rest was just a matter of tidying up.
Thought to be a deity of the ancient Britons rendered into a supposedly real, though obviously not an historical figure within a genealogy of British kings, Arthur turns out to be of greater antiquity than such an account presupposes. The name is usually understood to mean 'bear' or 'stone', yet there is no cogent basis to this. No sense can be made of either of those meanings to pertain to the hero of Arthurian legend. Trying to find any place where there is significant mythology centred on the bear is a wild goose chase until you travel to Switzerland.
The Welsh and Gaelic word for 'bear' is the single-syllable arth or art, leaving the second syllable unexplained, unless – it has been suggested – it is -(g)wr, 'man'. This is dismissed in a (surprisingly useful) Wikipaedia entry: "There are phonological difficulties with this theory -- notably that a Brittonic compound name Arto-uiros should produce Old Welsh Artgur and Middle/Modern Welsh Arthwr and not Arthur (in Welsh poetry the name is always spelled Arthur and is exclusively rhymed with words ending in -ur – never words ending in -wr – which confirms that the second element cannot be (g)wr 'man').
To try to address the impasse, a new etymology was tentatively proposed in 2009 by Stefan Zimmer (https://www.academia.edu/3255782/2010). His putative Brittonic construction Artu-rig-ios, from arto-rig, meaning 'bear- king', Zimmer admits is "nowhere attested in the Celtic world", but he proffers it as a name given to a Roman commander in Britain; this being the figure he supposes the source of Arthurian legend. A fanciful claim at best, it's a usual sort of attempt at trying to identify an historical figure for one which all but screams its mythological nature. In any case, this bid is a most convoluted etymology, and one in which Zimmer himself appears to lack confidence. It's evidently a straw-clutching exercise. In some etymologies the origin in 'Celtic' re a meaning of 'bear' is taken back to proto-'Celtic' artos, but there is no explanation etymologically of how the second syllable could change to produce arthur. A root in artos is offered in a database of Scottish names (available at http://www.amethyst-night.com/names/scotmale.html): "Artair — (AHR-shtuhr), 'eagle-like' or 'high, noble'; Gaelic form of Arthur, fr. Celtic artos 'bear', or poss. borrowed from Latin Artorius." The meaning here looks more promising, but it's entirely superficial, being merely derivative of the very construction at issue: the meaning appears to be a generic extension from the specific figure of King Arthur. That King Arthur self-evidently was 'noble' is not informative as to whether the notion of nobility is in the etymology of his name. Artair apparently here is a mere Gaelicisation of a word/name from another (Celtic) language. So it just takes us back to the usual suggestion of a meaning of 'bear'. In any case, a rendering into Gaelic would seem to have it backwards, in that the name was more common historically in Gaelic than in Welsh. A Gaelic origin looks the more likely; in which case the name underwent a transition from a Gaelic to a Welsh form and then to English, or, even (though, surely, unlikely), direct from Gaelic to English.
A trawl through Gaelic lexicons in conjunction with checking pronunciation throws up athaid, athach: 'monster'. The pronunciation suggests a feasible transition, and not only does a meaning 'monster' immediately arouse interest because of the clear possibility that it is an allusion to a former religion now eclipsed by one which necessarily portrays the religion it replaced as 'devil worship'; but it is decidedly interesting where this leads. It's fairly obviously a derivation from the Gaelic nathair, 'snake', used to denote 'serpent' or 'dragon'. This is precisely where the investigation into Robin Hood mythology led. The conceptualisation here is in terms of the weaving motion of a serpentine creature, with its being from Old Irish nathir, in turn from Proto-Celtic natir, related to snath ('thread'), snathad ('needle').The pronunciation is approximately 'narr-hurr', which seems not far from that of arthur, assuming that in Anglicisation the 't' would no longer remain silent. Evidence of such a change actually happening in transition to English is needed if this putative derivation is to hold water.
Just such seems to be provided in the place-name Athersley -- first recorded as Hattirslay -- a village by Barnsley in South Yorkshire. As I explained in the Robin Hood text, 'Celtic' and not least (if not especially) Gaelic roots of the stems of Pennine South Yorkshire place-names are commonplace – indeed, it's the more usual derivation rather than from even Anglo-Saxon. Corroborating evidence shows this particular place-name to be derived from nathair in a standard hybrid with a much later common suffix, nathair('s)-leah. What appears to have originated as nathair has undergone a transition through Anglicisation of a sounding of the 't' as written but silent in the Gaelic. The dropping of the initial 'n' presumably would have already occurred within a Gaelic-speaking milieu judging by the afore-mentioned athaid / athach.
Athersley formerly was a name attached to a wood (on which site is now the modern village), by an ancient holy well named St Helen's (on early OS maps, now giving its name to an area right by Athersley village). This is a generic naming of holy wells, with Helen (here formerly Ellen, and likely still earlier as elsewhere, Elian or Eilian, pronounced ehl-ihn) apparently derived from Gaelic Aillen, a mythological fire-breathing water-monster; a dragon, or serpent, indeed, with affiliation to water. [Wells were thought of by 'Celts' as interfaces with the 'otherworld', and as such were envisaged as being guarded by mythological creatures -- serpents. So the general category nathair clearly would pertain to St Helen's Well here. Confirming that this Well had Gaelic mythological association, adjacent to St Helen's Well on early OS maps is Smithy Hill, which is a generic naming of small knolls (as, for example, also in South Yorkshire at Stocksbridge) from Gaelic sithean, 'fairy spirits', who were considered to be of the 'inter-world', living in mounds.] The wood stretched along Carlton Road to Carlton Hill and village, which last was rendered as Carleton or Carlenton in Domesday; and therefore, as with nearby Cawthorne (recorded in Domesday as Caltorn), most likely derived from Gaelic caltuin, 'hazel grove', which is a generic feature of holy significance. The parent model for this derivation is the famous Carlton Hill in Edinburgh. It would appear, then, that just as is also evidenced in place-names at Cawthorne -- with its Serpent's Well -- there is in St Helen's Well and Carlton (Road, Hill and village) likewise a cluster of an ancient holy well and concrete allusions to a serpent and to a hazel grove. This cluster is a standard mythological three-way mutual association in Gaelic folklore. All of the evidence here is internally and externally consistent, and thus is fully explained the occurrence of a place-name for the locale with a derivation from nathair: Athersley.
Note that long ago a different derivation had been proffered for the Ather- stem -- though this is re surnames, not for the placename Athersley or similar -- by Henry Harrison in his Surnames of the United Kingdom: A Concise Etymological Dictionary. He took Ather- to be OE oedre, 'spring'; yet no trace is to be found anywhere else of this supposed word/meaning, which anyway begs questions regarding the sound transitions that would be needed for the etymology to be accurate. It seems, therefore, that here we have a mistake by or invention of Harrison's. As his compilation was published back in 1918, when 'Celtic' generally, never mind Gaelic roots of English names normally would not have been considered, leading to 'forced' derivations from a restricted lexicon (and in this, still today the English Place Name Society is notably remiss), then this may be what Harrison did here. In any case, etymology was at that time lacking rigour, being far more like mere guesswork than it is today. Poor scholarship, then, would appear to be the explanation for this wayward attempted derivation.
As for other attempts at derivation -- of the placename: they have been poor even by the low standards of placename research in England. Regarding also the possibly related Hattersley (Cheshire) but forgetting Atherton and Athelstone, which appear to have quite different roots; there is the usual feeble recourse to a supposition that the stem is from some Anglo-Saxon first name. The dire state of the specific etymology is starkly admitted by John McNeal Dogson: 'The -er- in Hattersley Cheshire and Hothersall Lancashire'. Leeds Studies in English 18 (1987) http://digital.library.leeds.ac.uk/256/1/LSE1987_pp135-39_Dodgson_article.pdf
Looking across language use and mythology, there is decisive support for the argument that nathair indeed is the derivation of (King) Arthur, in that the Gaelic compound word righ-nathair – literally 'king-serpent' – is the term for 'cockatrice', 'dragon'; exactly corresponding to the meaning of Arthur's supposed surname, as that of his father in the Welsh tradition: Uther Pendragon (from the Welsh pen-, 'chief').
It's hard to believe that no-one hitherto had come up with this really quite simple analysis to account for the origin of King Arthur's name. It must be, just as in the case of Robin Hood, that with a failure to fully appreciate the antiquity of much mythology, then there has been no consideration that quintessentially English figures would have even the remotest connection to Gaelic -- especially as any cursory look would be dissuasive in the absence of an appreciation of the quite marked sound changes across the language transition to English. Yet none of these factors should apply to scholars, so where have they been? Apparently, the same place they often are: in 'groupthink' misplaced deference to 'authority', protecting their own position by avoiding the risk of stepping out of line, instead of thinking laterally, putting in some work and doing their job.
From the research into the etymology of the Robin Hood name (see the parallel investigation on this site), we know that 'serpent' is an epithet of the pan-'Celtic' primary deity, Bridhe (Brigid). [Rather than reproduce all of the research into this here, it is perhaps best left in the context of the far more extensive etymological excavation that is the Robin Hood paper.] Arthur, then, is not merely akin to Robin Hood in being of Gaelic origin, but appears to be a manifestation of the very same 'earth' goddess of regeneration -- that is, he is the 'sacred king' figure who self-sacrifices to the 'earth' goddess; so that he takes her name whilst being dubbed 'king'. This is hardly unexpected given the centrality in Arthurian legend of the 'grail' – another name for the mythical inexhaustible cauldron of regeneration lore; the receptacle of the sacred king's self-sacrificial blood, spilled in homage to the deity with the supposed function of bringing renewed life to the land so as to continue the life/death/rebirth regenerative cycle. [The Arthurian stories in outline are familiar to everyone, and there is much writing on them without requiring any from me.]
That they appear to us as entirely separate mythologies indicates that the two traditions originated from different cultures separated by language, which later came together through the eclipse of 'Celtic'-speaking regions in the Western half of England as English became ubiquitous. King Arthur being still more obscure than Robin Hood suggests that Arthur may be the more ancestral figure of the two. On the other hand, whereas Robin Hood seems to be ambiguous as to whether he is the 'red king' self-sacrificial 'sacred king' or the deity to which this self-sacrifice is made; King Arthur is clearly distinguished as the former. It's not clear if this clarity indicates a more recent origin for Arthurian than for Robin Hood mythology. Either way, the profundity of the notion of the life/death/rebirth never-ending cycle of regeneration as being the core of mythological imagination of old -- as, surely, it must remain in some form or other even today -- is pointed up by this surprising convergence of these two key English legendary personages.