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"Break a leg" is a well-known idiom in theatre which means "good luck." It is typically said to actors and musicians before they go on stage to perform. The origin of the phrase is obscure.

The expression reflects a theatrical superstition in which wishing a person "good luck" is considered bad luck. The expression is sometimes used outside the theatre as superstitions and customs travel through other professions and then into common use. Among professional dancers, the phrase "break a leg" is replaced with "merde".

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