Jersey Bet

When one proposes an absurd, audacious, undesirable, or illegal act to another, and follows it with the words: "Do it. You won't". The second person is obliged to complete said act.

Guy: See that girl over there? You should go make out with her.
Girl: No way! I'm straight.
Guy: DO IT. YOU WON'T.
Girl: Okay, fine, I'll do it.
Guy: Gotta love a Jersey Bet.