Is it ensure, insure, or assure?
late 13c., from Old French obeir "obey, be obedient, do one's duty" (12c.), from Latin obedire, oboedire "obey, be subject, serve; pay attention to, give ear," literally "listen to," from ob "to" (see ob-) + audire "listen, hear" (see audience). Same sense development is in cognate Old English hiersumnian. Related: Obeyed; obeying.