Break a Leg

Alright, listen up, buttercup. This joke is dark. John Wilkes Booth was an actor who assassinated Abraham Lincoln at Ford's Theatre. His fiancee tells him to "break a leg", which is something people say to actors to wish them good luck. However, in this context, it's darkly humorous because Booth literally "broke a leg" of American history by killing Lincoln. You probably would have preferred to play with your poop.