meaning: Good luck! or wishing someone a success
example:
Me: I have a job interview tomorrow.
You: I know you can do it. Break a leg!
origin:
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.
(http://www.phrases.org.uk)
