Is it ensure, insure, or assure?
c.1300, from Old French espondre "expound (on), set forth, explain," from Latin exponere "put forth, explain, expose, exhibit," from ex- "forth" (see ex-) + ponere "to put, place" (see position); with intrusive -d (cf. sound (n.1)). The usual Middle English form was expoune. Related: Expounded; expounding.