‘Break a leg’ means ‘good luck’ (often said to actors before they go on stage).
Examples:
“Break a leg Sam, I’m sure your performance will be great.”
Or
“You have an exam tomorrow? Break a leg!”
The Origins
This phrase has its origin in the world of theater. Performers had a superstition that saying “good luck” would actually bring them bad luck, so “break a leg” was used instead.