"Break a leg" is a well-known saying in theatre which means "good luck". It is typically said to actors before they go out onto stage to perform.
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
Break a leg - Wikipedia, the free encyclopedia
Said to actors for good luck before they go on stage, especially on an opening nights.
Theatrical types are well-known for their belief in superstitions, or at least for their willingness to make a show of pretending to believe them. The term 'break a leg' appears to come from the belief that one ought not to utter the words 'good luck' to an actor. By wishing someone bad luck, it is supposed that the opposite will occur.