Time Traveler. I had always heard of break a leg as in 'bend a knee,' apparently a military term. Seemingly this gave rise to the English expression, which according to Brewer was still in use at the end of the s 'He may fetch a flitch of bacon from Dunmow' a flitch is a 'side' of bacon; a very large slab , which referred to a man who was amiable and good-tempered to his wife. The main usage however seems to be as a quick response in fun, as an ironic death scream, which is similar to more obvious expressions like 'you're killing me,' or 'I could scream'. Further to the above entry I am informed thanks Dr A Summers, Mar of another fascinating suggestion of origin: "
nest...