The action or act of breaking a person's or animal's leg or legs; the act of breaking one's leg.
Early 19th century; earliest use found in Belle Assemblée. From leg + breaking.
That breaks or is liable to break a leg or legs. Also in extended use: arduous; treacherous.
Mid 19th century; earliest use found in Walter Colton (1797–1851). Partly from leg + breaking, and partly from leg break + -ing.