c.1320, of persons, "diseased, wounded," from un- (1) "not" + sound (adj.). Cf. M.L.G. unsund, M.Du. ongesont, Ger. ungesund. Meaning "morally corrupt" is recorded from c.1300; that of "not mentally healthy" is from 1547. Sense of "not based on reasoning or fact" is attested from 1595.