King Arthur is a well known figure. Much of what is written about him is made up, but there is a strong possibility that he was indeed an early king of the Britons first recorded in a work attributed to Nennius. Like so many other early figures, his name is assume to be Welsh. Does this hold up? Is there a better etymology in Old English?
This one of the simplest and most obvious etymologies:
Old English words: ar ('Awe', honour, glory) and Thor (Thor the Norse God)
Which together would be translated as "glorious Thor".
It's that simple. In the same way we have "Christopher" derived from Christ, so it would seem Arthur's name could be derived from Thor.
And the prefix Ar- is widely used with similar words like:
ar-craeftig ( skilful in respect, polite) craeftig = skillful.
ar-ful ( full of honour)later full of "awe", or something to be respected and thence "awful"
ar-faet (A brazen vessel) faet a plate.
ar-fest (merciful) fest = firm
ar-hwaet (bold) hwaet=eager, so eager of honour.
Compare to the supposed "Celtic" etymology:
"from Welsh arth "bear,"
But what about Latin:
Latin artorius ‘plowman’.
But note how all kinds of possibilities are considered apart from the obvious one of Old English:
It has been argued that the name Arthur is derived from 'bear', which corresponds with Celtic bear gods Artos or Artio.
The origins of Arthur's name have been subject to much debate throughout the ages. Some believe it to have derived from Lucius Artorius Castus, a second century Roman officer based in Britain.
Others believe that the name has Welsh origins. The fifth century Welsh word art means 'bear'. One theory is that art and ur, meaning 'man', gives us the name Artur - literally, bear-man. This is supported by late Latin writings which give the name Arturus.
Other variants have been put forward, including Arturius, Arcturus, Arturos and Artursus. The latter name combines the Welsh for bear - art - along with its Latin counterpart - ursus.(BBC)