Etymology

Several theories exist as to the origin of this phrase. All are heavily debated. See Wikipedia: Break a leg.

Verb

1. A wish for a successful performance; primarily a valediction to an actor wishing him or her a successful theatrical stage performance.
2. To perform well in a theatrical production or comparable endeavor.

   Go out there and break a leg tonight. Put on a great show!

Usage notes

Always in the exact form "break a leg" for the "perform well" sense, usually an interjection in and of itself. Sometimes used in a verb sense ("Go out there and break a leg"), but always in the imperative. Any other verb tenses such as "she broke a leg last night" or "we were breaking legs out there" would not carry the same meaning.