It has been thought that the name "Arthur" derived from the Roman "Artorius" although most scholars claim that the name "Arthur" is unknown in Britain prior to the late-sixth century. There was one notable exception:, Lucius Artorius Castus, who lived and fought in Britain in the late second-century.
The name "Arthur" started cropping up among late-sixth-century and seventh-century Irish migrants to Wales and Scotland That people would suddenly start naming sons "Arthur" and literature recording "although he was no Arthur" (the reference to Arthur in Y Gododdin, ca. 600), indicates that the name Arthur name had become fairly common by this time.
The Historia Brittonum (ca. 800),15 which was probably compiled by, rather than written by, Nennius, is the oldest work to record legends of Arthur. By the beginning of the ninth century, Arthur was known as both a dux bellorum and a miles ("soldier"), although the legends of Arthur being a king were apparently already in circulation, given the care with which Nennius points out that Arthur was not a king.
"Arturus" is what is found in the earliest Latin reference of Nennius and Geoffrey of Monmouth (1844;157). With either form though, we already have a word that would have been recognized by any of Arthur’s Latin-speaking contemporaries in beleaguered Britain.
Scholars point to the derivation Latin artorius ‘plowman’. To call Arthur a plowman, does not seem probable.
Arcturus is the brightest star in the constellation Bootes. Known as the "Bear Watcher," it follows Ursa Major, the Great Bear, around the night sky.
This star is regarded as the leader of the rest and the name Arcturus means
the ‘guardian of the bear’. In the case of "Arthur"
this could have referred to a bear or to a human with bear-like attributes.
A Briton hearing the word Artur (or possibly Arturos with a nominative masculine ending) would have heard someone saying the equivalent of English Bear-man. A suitable name for a British leader fighting the Saxons.
So you can take your pick as whether the name "Arthur" came from
a Latin root or from the stars. There is no proof one way or the other.
Arthur, who was he?