Arthur likely comes from 'art' meaning bear combined with 'viros' meaning man, or it could be related to Irish 'art' meaning stone.
So... when you take the nickname Art from Arthur, and actually name your child Art, does the name still mean 'bear man' or w/e, or just 'bear' now. Because the name Art wasn't derrived from 'art' to mean bear, but from a name meaning bear man... does art, which is the word for bear, really mean bear man? Kinda philosophical eh? What's in a name newho:P