Posted by Mike on May 15, 2004
Although nobody can be really sure where the phrase originates, the most likely place (at least for the Theatre World) is from the most unlikely of items: The curtain.
To avoid confusion between what is real and what is 'real' to the play, Theatre has its own terms for many items with trucks, flies/fly's, and flats are some of the more common. The main curtain that seperates the stage from the audience is know as the 'Tabs', and the rope used to lift and lower it was archaically known as the 'Leg'.
To wish that someone would "Break The Leg" was to suggest that they had so many "curtain calls" to take more and more bows that this rope would literally fray and break. So to say "break a leg" is to wish someone an impecable performance rather than strictly good luck.
It has been suggested to me that this may be linked to a 'leg' of a match. That one 'leg' is up to the point that the leg-rope was used to bring the curtain in, ie: the first half of a show. I'm not sure how true this part is though.