Arthur m. Of uncertain origin, but likely connected to Old Breton arth, Proto-Celtic *artos, Greek ἄρκτος 'bear'.
The popularity of this name in the Middle Ages is directly attributable to the semi-legendary 5th-6th C British king whose exploits are found in both medieval historical and literary (esp. poetical) sources. The Arthurian tradition of knightly virtue and chivalry spread from England to France via Brittany, and from there to the rest of Europe.