It is almost certain that Arthur is a Celtic form of the Roman Artorius, which belonged to several Roman figures and to a gens, or clan. It was known in Roman Britain as early as the second century: a officer named Lucius Artorius Castus led a military excursion into Brittany around 150 AD.
Certain scholars have proposed alternatives to this theory. E.W.B. Nicholson regards Arthur as a combination of two Celtic words: artos (“bear”) and viros (“man”), which would make Arthur’s name a metaphor for a strong warrior: “Bear-Man” (Welsh: arth gwyr).
A. Holder proposed a connection with the Irish art (“stone”), and M. Müller tried to link Arthur to a culture-divinity in part by noting that the Aryan term for “to plough” is ar. None of these theories have won wide acceptance.