Break a Leg

Genuinely one of the hardest idioms to pin down. The earliest confirmed use in print comes from American writer Edna Ferber's 1939 memoir, and anecdotal accounts suggest actors were already saying it in the 1920s and 30s, but nobody has found where it started. Leading theories include a superstition that wishing someone luck directly would jinx them, so an ironic, backwards-sounding phrase was used instead, and a possible echo of the German theatrical phrase "Hals- und Beinbruch" (neck and leg break). A popular story linking it to John Wilkes Booth breaking his leg after assassinating Abraham Lincoln on a theatre stage in 1865 is almost certainly false, since the phrase isn't recorded anywhere until decades later.