"Break a leg!"

     [This is a fast-access FAQ excerpt.]
There is a superstition in the theatre that wishing an actor
good luck "tempts the gods" and causes bad luck, so negative
expressions are substituted.  In French one says Merde! ("Shit!")
when an actor is about to go on stage.  The German expression is
Hals und Beinbruch="neck and leg fracture" (Bein used to mean
"bone" in German, so the translation "neck and bone break" may be
correct if the expression is sufficiently old).  The leading
theory is that the English expression came from the German, possibly
via Yiddish.  Other suggested origins are:  John Wilkes Booth, the
actor who broke his leg shortly after he assassinated Abraham
Lincoln in 1865; the great French actress Sarah Bernhardt, who "had
but one leg and it would be good luck to be like her"; wishing
someone a "big break", that is, good luck leading to success; and
the Hebrew hatzlacha u-brakha = "success and blessing".