You're the fall guy if you look guilty and are punished for a misdeed when you're actually innocent. A fall guy is often foolish or trusting enough to be taken advantage of, and it's a common character in movies and books. Fall guy is thought to have been first used in the US around the turn of the twentieth century, but its origin is disputed. It most likely comes from a criminal slang meaning of fall — "arrest" or "prison term."