Break a leg

Altered image of a person with a broken leg
Break a leg is a phrase used in the theater to wish a performer good luck before a show.

The expression became common in English-speaking theater communities during the 1900s. One explanation is that performers avoided saying "good luck" because it was considered bad luck, so they used a joking opposite phrase instead.

Just before the curtain went up, her friend whispered "break a leg".