This English idiom is commonly used as a metaphor. Surprisingly it is actually a positive statement although you would be forgiven for thinking otherwise!
This expression is used in order to wish someone good luck in something. It can relate to acting as it was originally intended to do but it is now also commonly used before exams, interviews or any important event.
The first recorded usage of this expression is said to date back to the acting world in Great Britain in the 1920s. It is suggested that it originated as a superstitious ritual among actors and actresses. The idea of “breaking a leg” was never meant in a literal sense rather it is believed that the idea of “breaking” was related to the act of being exciting, energetic and giving a good performance.
The first written record of the idiom also dates to the 1920s in a 1921 edition of The New Statesman. In this Robert Wilson Lynd wrote “you should say something insulting such as, may you break your leg!.”
An example of the usage of this expression can be found in the 1939 work A Peculiar Treasure by Edna Ferber. In this she writes “and all the understudies sitting in the back row politely wishing the various principals would break a leg”.