Break A Leg
As is often the way with language, there seems to be no definitive answer as to the true derivation of this term.
Below are offered some suggestions:
1) Lincoln / Wilkes Booth Injury
From Dave Wilton's Etymology Page : http://www.wordorigins.org/
Superstition against wishing an actor Good Luck! has led to the adoption of this phrase in its place. Popular etymology derives the phrase from the 1865 assassination of Abraham Lincoln. John Wilkes Booth, the actor turned assassin, leapt to the stage of Ford's Theater after the murder, breaking his leg in the process. The logical connection with good luck is none too clear, but such is folklore.
There is no evidence, however, to suggest that this is the true derivation, and since the earliest usage of the phrase dates to the 1920s, there is much to suggest that it is not. The best that can be said is that the origin is unknown.
2) Hals Und Beinbruch
A DICTIONARY OF CATCH PHRASES (see below) suggests that there may be a connection with the German phrase Hals und Beinbruch, an invitation to break your neck and bones. The German phrase is used by aviators and is equivalent to the English phrase Happy Landings!. Both phrases arose about the same time, the early twentieth century, but the connection between the German aviation community and American theater is unclear, so they may be unrelated.
A Dictionary of Catch Phrases; Eric Partridge; edited by Paul Beale; Scarborough House; 1992; ISBN 0-8128-8536-8. Contains excellent information, but unfortunately suffers from an odd alphabetization system, the lack of an index, and few cross references, all of which makes finding the phrase you want difficult at best.
A DICTIONARY OF SLANG AND UNCONVENTIONAL ENGLISH, published some eight years before the above, does not list the theatrical meaning. Instead, it lists an obsolete meaning of "to give birth to a bastard child," from circa 1670.
A Dictionary of Slang and Unconventional English, 8th Edition; Eric Partridge; edited by Paul Beale; MacMillan; 1984; ISBN 0-02-594980-2. A superb source that focuses mainly on British slang, but which is also useful for Americanisms.
For more interesting word origins, visit Dave Wilton's Etymology Page : http://www.wordorigins.org/
3) Bending the Knee
From : http://www.footlights.com/art7d.html
For contemporary English-speaking theater people, the ritual greeting reflects that calamitous 42nd Street production, "Break a leg."
However, the rather terrible curse may have had a more benign origin. Much earlier in stage history, when superstition had a less frightening hold on the craft, actors and their followers used a more gracious greeting: "May you break your leg," by which it was meant that the evening's performance would be of such grandeur that the actor would be obliged to break his leg - that is bend his knee - in a deep bow acknowledging the audience's applause.
4) Getting Onto the Stage
From : Josh Pritchard - firstname.lastname@example.org
Evidently, in the days of early vaudeville, the producers would book more performers than could possibly perform in the given time of the show - since "bad" acts could be pulled before their completion... so, in order to insure that the show didn't start paying people who don't actually perform, there was a general policy that a performer did NOT get paid unless they actually performed on-stage. So the phrase "break a leg" referred to breaking the visual plane of the legs that lined the side of the stage.
i.e. "Hope you break a leg and get on-stage so that you get paid."
5) Outsmarting the Sprites
From : http://members.aol.com/morelandc/HaveOriginsData.htm#BreakALeg
Meaning: A wish of good luck, do well.
Example: Break a leg in your game today.
Origin: "Break a leg" is sourced in superstition. It is a wish of good luck, but the words wish just the opposite. It was once common for people to believe in Sprites. Sprites are actually spirits or ghosts that were believed to enjoy wreaking havoc and causing trouble.
If the Sprites heard you ask for something, they were reputed to try to make the opposite happen. Telling someone to "break a leg" is an attempt to outsmart the Sprites and in fact make something good happen. Sort of a medieval reverse psychology. Of course it has became a popular wish of luck for theater performers.
6) Take A Bow
From Gary of Players Ring :
7) Break A Legend
From David Scears :
In the nineteenth century theatre, when it was the norm for actors like Keen, Tree, and Irving to be actor managers. They would perform a role many times and for many years. When a new actor would take over a particular role that had become closely associated with one of these legendary actors he was told "break the legend". Over time this gradually got changed to "break a leg".
8) Greek Stomping
9) Elizabethan Stomping
10) Compensating for Injury
11) Australia: 'Chookas'
From Naomi Guss
The Australian term for "break a leg" is "chookas" (pronounced chook-as).
There are many theories for this, but I can't remember any at the moment!
12) Opera Singers
Opera singers use 'Toi Toi Toi' which is believed to be an onomatopoeic representation of spitting three times (believed to expel evil spirits)
Thanks to Adrienne Redd for help in locating definitions 1 to 5
and to the folk who've taken the time to email in their own suggestions.