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"mental inclination," 1570s, probably from earlier literal sense "condition of being deflected or turned" (1530s), from bent (adj.) "not straight" (q.v.).
"stiff grass," Old English beonet, from West Germanic *binut- "rush, marsh grass" (cf. Old Saxon binet, Old High German binuz, German Binse "rush, reed"), of unknown origin. An obsolete word, but surviving in place names (cf. Bentley, from Old English Beonet-leah; Bentham).
"not straight," 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. Figurative phrase bent out of shape "extremely upset" is 1960s U.S. Air Force and college student slang.