The idiom 'Break a leg' is usually used in theater to wish good luck to actors before they go up on stage.
Example of use: "Danny's family told him to "break a leg" right before he went up on stage."
Interesting fact about Break a Leg
The expression “break a leg” has been common among actors since the early 1900s and presumably reflects a theatrical superstition in which wishing a person "good luck" is considered bad luck. There are some who argue that it originated when John Wilkes Booth broke his leg after assassinating President Lincoln in Ford’s Theatre.
