BREAK A LEG?
Some actors used to be, and still are, very supertitious, Therefore, instead of saying ''Good luck'' (they thought they would jinx themselves if they did this) they would say ''Break a leg''. Though they really meant to say ''Good Luck''.
There is a superstition in the theatre, and other performing arts, that to wish someone "good luck" before a performance will bring bad luck! Instead, it is customary to say "break a leg!"
The 'leg' is part of the curtain that comes up and down after a finale. Extreme use of the 'leg' could cause it to break. When the audience carry on clapping and shouting after the finale in a show, the curtain comes back up and then down again, too much of this action could cause the 'leg' to break.
The expression "break a leg" means to do well at a show. The curtain itself is big so it is supported by different mechanics. These mechanics rest on "legs". At the end of a show, the actor comes out for an ovation (hopefully). The curtain opens and closes every time there's an ovation. So, saying break a leg means to have the audience keep clapping for ovations so much that the curtains legs will break.
Two other possibilities; The word 'break' has many meanings - the OED lists 57 distinct uses of it as a verb alone. That gives considerable scope for speculation over what is meant by the phrase. The most common interpretation of 'break' in this context is, 'to deviate from a straight line', as in the cricketing term, 'off break'. That is, unstraighten the leg by bending at the knee, by bowing or curtseying.
Or perhaps it comes from a German saying, 'Hals und Beinbruch', meaning 'break your neck and leg', which dates back to at least WWII, as Luftwaffe slang, and is therefore earlier than any known English version. It may be that this is a corruption of the Hebrew blessing, 'hatzlakha u-brakha', meaning 'success and blessing'.
