Break a legg... Waarom zeggen ze dit in Amerika als men iemand succes wenst?

even te lui om te vertalen..

Superstition against wishing an actor Good Luck! has led to the adoption of this phrase in its place. Popular etymology derives the phrase from the 1865 assassination of Abraham Lincoln. John Wilkes Booth, the actor turned assassin, leapt to the stage of Ford's Theater after the murder, breaking his leg in the process. The logical connection with good luck is none too clear, but such is folklore.

There is no evidence, however, to suggest that this is the true derivation, and since the earliest usage of the phrase dates to the 1920s, there is much to suggest that it is not. The best that can be said is that the origin is unknown.

A DICTIONARY OF CATCH PHRASES (see below) suggests that there may be a connection with the German phrase Hals und Beinbruch, an invitation to break your neck and bones. The German phrase is used by aviators and is equivalent to the English phrase Happy Landings!. Both phrases arose about the same time, the early twentieth century, but the connection between the German aviation community and American theater is unclear, so they may be unrelated.

A DICTIONARY OF SLANG AND UNCONVENTIONAL ENGLISH, published some eight years before the above, does not list the theatrical meaning. Instead, it lists an obsolete meaning of "to give birth to a bastard child," from circa 1670.

Een andere betekenis komt van het engelse woord voor zijdoek ("poot"): leg.
Wanneer je opkomt op het toneel kom je langs dit doek; het zou dus synoniem zijn voor "je mag op".
Uiteindelijk weet niemand waar het vandaan komt..