break a leg

Unknown; many unproven and widely debated theories exist. One of the most plausible is that it comes from Yiddish הצלחה און ברכה (hatslokhe un brokhe, “success and blessing”) through the heavy Ashkenazi Jewish influence in the American theater, via the misinterpretation in German as Hals- und Beinbruch (“neck and leg fracture”). The Yiddish phrase itself comes from Hebrew הַצְלָחָה וּבְרָכָה (hatzlakhá u-v'rakhá, “success and blessing”). Another possible origin was to wish opposite luck on the performer so as not to jinx the statement.

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break a leg!

1. (idiomatic) Said to someone wishing they perform well in a theatrical production or comparable endeavor.

   Synonym: (opera) toi, toi, toi

   Go out there and break a leg tonight. Put on a great show!

   I told my friend to break a leg, before she went on stage.

• Michael Quinion (2004) “Break a leg”, in Ballyhoo, Buckaroo, and Spuds: Ingenious Tales of Words and Their Origins, Washington, D.C.: Smithsonian Books in association with Penguin Books, →ISBN.
• Gary Martin (1997–) “Break-a-leg”, in The Phrase Finder.
•   break a leg on Wikipedia.Wikipedia
• Break A Leg - English Idiom