Is it ensure, insure, or assure?
c.1300, from Old French assiginer (13c.) "assign, set (a date, etc.); appoint legally; allot," from Latin assignare "to mark out, to allot by sign, assign, award," from ad- "to" (see ad-) + signare "make a sign," from signum "mark" (see sign). Main original use was in English law, in transferences of personal property. General meaning "to fix, settle, determine, appoint" is from c.1300. Related: Assigned; assigning.