What does "break a leg" mean — and why is it funny?

informal, theatrical

Meaning

Good luck — especially said to a performer before they go on stage.

Where it comes from

A theater superstition: actors believed that wishing someone 'good luck' directly would jinx the show, so they wished the opposite instead.

Why it is funny

The comedy is in the gap between the literal words and the warm intention. You appear to be wishing someone a serious injury, but everyone in the room understands it as affection. Outsiders hearing it for the first time are briefly alarmed, and that flicker of alarm is the joke.

Used in a sentence

"You're going to be great out there — break a leg!"