Is it ensure, insure, or assure?
Old English uncuð "unknown, uncertain, unfamiliar," from un- (1) "not" + cuð "known, well-known," past participle of cunnan "to know" (see can (v.)). Meaning "strange, crude, clumsy" is first recorded 1510s. The compound (and the thing it describes) widespread in IE languages, cf. Latin ignorantem, Old Norse ukuðr, Gothic unkunþs, Sanskrit ajnatah, Armenian ancanaut', Greek agnotos, Old Irish ingnad "unknown."