Someone who has an invisible, permanent "Kick Me" sign taped to their back.
They're the butt of almost any demeaning joke, they commonly have horrible luck, and a large amount of horrible things happen to them. Their existence is just to solely have horrible things happen to them. If anything positive happens to them, the person usually gets screwed over in the end.
Charlie Brown from Peanuts is probably the best example for a "Butt Monkey".
Bill Dauterive from King Of The Hill.