Someone who writes Urban Dictionary definitions about themselves, or people who they know, but who are not famous. Esp. when those definition is "*insert name here* is very cool and attractive."
Person 1: Hey! Jeremiah defined himself on Urban Dictionary!
Person 2: What an Urban Dictionary Dumbass...
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.