"not straight, curved like a strung bow," late 14c. (earlier ibent, c. 1300), from past participle of bend (v.). Meaning "turned or inclined in some direction" is from 1530s, probably as a translation of Latin inclinatio. Meaning "directed in a course" is from 1690s.
Used throughout 20c. in various slang and underworld senses: "criminal; illegal; stolen; corrupted; broken; insane; homosexual;" compare slang uses of crooked. Figurative phrase bent out of shape "extremely upset" is 1960s U.S. Air Force and college student slang.