This means "good luck." It is often used to wish someone success before a performance or a challenging situation, especially in the context of theater, sports, or any kind of public performance.

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Idiom

Break a leg

Narrator: "For example, if a friend is about to go on stage for a play, you can say, 'Break a leg!' to wish them good luck."

Narrator: "Though 'break a leg' might sound negative because of the words 'break' and 'leg,' it is a positive way to wish someone luck in English."