Is it ensure, insure, or assure?
early 14c., of persons, "diseased, wounded," from un- (1) "not" + sound (adj.). Cf. Middle Low German unsund, Middle Dutch ongesont, German ungesund. Meaning "morally corrupt" is recorded from c.1300; that of "not mentally healthy" is from 1540s. Sense of "not based on reasoning or fact" is attested from 1590s.