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.
A Butt-Monkey is someone who worships another person. They laugh at all their jokes whether they're funny or not, they think they're great even when they're not and generally strive to spend as much time with that person as they can. The person the Butt-Monkeys follow is called a Butt-Monkey King or Queen.