With this article, I’ve heard another theory which wasn’t mentioned on in the post…
This page has a number of potential sources. The second one is the one that I’ve heard before, and seems the most plausible to me:
Is there any evidence for the use of the phrase “break a leg” in its meaning of “bend the knee” in early English? I see nothing in the OED first edition that would indicate break to mean bend in any similar sense.
Without an earlier citation, it’s extremely unlikely that the “bend the knee” interpretation is anything but folk etymology. Besides, it’s been established that the first use of the term is print is as late as 1957, with some memories of its use going back to the 1920s. That’s far too late for such an old-fashioned phrase to suddenly appear. Sorry.
Saying “good luck” is notoriously bad luck, so “break a leg” is like the opposite; a sly way of saying “good luck.”
The explanation I’ve heard, which I can’t provide a cite for, is that the curtains on either side of the stage were called “legs” (bein?) and the rods holding them were called the “neck” (Hals?) so that if the applause was thundrous enough their supports would break, leading to the (very real) German phrase “Hals und Beinbruch” (“neck and leg break”) which was translated into the American theater without explanation. No idea if it’s true.
Of course I like the J.W. Booth explanation as well.
I’ve never heard that explanation.
It seems questionable to me; it suggests the phrase was invented in German theaters and was copied by German aviators, who then brought it to English theaters. That seems unnecessarily convoluted; surely it could have gone from German to English theater more directly.
I was told by a Russian woman that she had learned the equivalent phrase in … was it in Czech? … was “don’t break your leg.”
This is my understanding as well. Sorry I have no cite, but here’s what I heard from other players when I used to do community theatre:
The gods of theatre would visit bad luck upon an actor who was wished good luck. (They’re kind of mischievous, apparently, and like to make sure that the opposite of what’s being wished occurs.) So, to confuse them, actors would say “break a leg” as a way of wishing good luck to each other. Confused, and wanting to do the opposite of the wish, the mischievous theatre gods would make sure the performance went smoothly, and the actors delivered great performances.
This earlier thread on the column raises the stage legs theory, but it gets shot down pretty conclusively.
. . . They drink a lot, too.
This story, according to The Word Detective, started out with German aviators, then the German theater adopted it, subsequently going into English theater from there. He also says that German aviators were using “Hals- und bienbruch” as a roundabout way of wishing good luck.
I think someone has confused the genuine old expression “make a leg” with “break a leg”.
The “bad luck for good luck” hypothesis tends to be confirmed by the operatic equivalent, “nella bocca del lupo”, “into the mouth of the wolf” (the frequent use of which I can personally testify to).
In France, I’m told, the other actors spit on you just as you’re going on stage and say, “Merde.”
Sadly, no cites.
They also say “in bocca lupo” before exams, to which the appropriate response is “crepi” (“may it [the wolf] die”).
I’d be interested in seeing a cite re the spitting custom. I’m only aware of “le mot de Cambronne” (merde) being used to wish good luck in the theatre. It is also used to offer wishes in other occasions (exams, job interviews and such).
I Googled on it before posting, but to no avail.
I used to do a lot of community theater and learned the tale from other actors.
The spitting was mostly pretend.
Stick out your tongue just a little. Withdraw it quickly and at the same time expel a short burst of air through your pursed lips.
Voila!
When my wife left to take her state exam for the CPA license, I remembered the theater superstition. I told her, “Break a pencil.”
Did it work? 
That’s only for American actors. 
Yeah, my mom always tells me that before a test, too.
Yup, she got her license. 
Don’t have any better info on the origin of the phrase. Just saw the Nathan Lane-Matthew Broderick movie The Producers, though, and the lead characters insist on saying “Good luck!” to their horrified castmembers just before “Springtime for Hitler” opens - because they want the production to fail, and to fail badly. Little do they know…