Break a leg!

 good luck

If actors were not performing, they had to stay behind the “leg line,” which also meant they wouldn't get paid. If you were to tell the actor to “break a leg,” you were wishing them the opportunity to perform and get paid. The sentiment remains the same today; the term means “good luck, give a good performance.”

The phrase is believed to be rooted in the theatre community, which is known to be a bit superstitious. Performers believed saying “good luck” would actually bring bad luck on stage, so they'd tell one another to “break a leg” instead. That way, the opposite would happen.