What you tell someone about to perform in a drama or musical performance to wish them well.
What you don't say is "good luck".

a well wishing phrase used in the theatre, and meant to be understood as "good luck." "good luck" is not used itself because it is believed to bring bad luck.

also, it can be a good luck gift given to an actor before a performance.

From the beginning a term used to wish people in Show-buisness good luck befor they entered the stage. Nowerdays a more spread term to wish anybody good luck.

Break a leg!: Good luck!

a saying used in theater to imply "good luck" in a way that seems like they want you to be injured; usually jinxes the recipient to get injured during the preformance

Often said to an actor before an audition by his/her mis-informed peers.
They belive that if he/she does break their leg they will be in a cast for weeks.
Idiots