Is it ensure, insure, or assure?
Old English gebecnian (West Saxon beacnian) "to make a mute sign," derivative of beacen "a sign, beacon," from Proto-Germanic *bauknjan (cf. Old Saxon boknian, Old High German bouhnen), from PIE root *bha- "to shine" (see beacon). Related: Beckoned; beckoning. The noun is attested from 1718, from the verb.