Break A Leg
This phrase dates back to the 1920's and is superstition against wishing an actor good luck. Many people think the origin comes from when in 1865 John Wilkes Booth, who was an actor, broke his leg while leaping to kill President Linoln during a play at the Ford's Theatre. But, this does not really seem like it is related to good luck. Some stage actors think it has to do with bending your knee when you bow, like at the end of a successful play.

A site visitor wrote the following reguarding this idiom:
In a short view of your idiom site came up with a couple of errors at once. Break a leg---Booth didn't jump to kill Lincoln, Booth, having shot Lincoln, jumped to the stage to escape and in doing so broke his leg.

This from another website visitor:
Break a leg is cited as superstition against good luck, which is what I originally thought it meant, too. Recently, however, I was informed that "leg" refers to the side curtains and it suggests that the applause ought to be so great that the legs fall break from the stage. I can't say how truthful this necessarily is. The side curtains are, however, called legs, so it's entirely possible.

This from another website visitor:
i would like to make a comment about "break a leg." The original origins of "break a leg," began in theatre ballet productions where they came out for curtain calls. The rod that allows for the curtain to be raised and lowered is called the "LEG". So "break a leg," means truly to get so many curtain calls that the leg breaks in two. Might want to put that out there and correct your site. (来源：最老牌的英语学习网站 http://www.EnglishCN.com)

This from another website visitor:
I'm heavily involved in theatre, and so have heard of several theories of the origin of the term. Here is the most persistent one that I hear: Any time a person or an object moves through or past a drop, they are said to be "Breaking Curtain." The thin curtains that mask the wings are referred to as "legs" Therefore, to enter from the wings, an actor must first "Break a leg."Back in the day, particularly in the era of vaudville, variety shows consisting of assorted talent acts were fairly common, and often the
crowd's reaction led to a person being either allowed to finish their act, or forced off the stage, but even a few minutes in the limelight was good exposure for an actor. Ergo, even if their performance wasn't great, or if the actor was shot down before he got a chance to perform, he still had managed to get his name and face out there by
breaking the legs.