Is it ensure, insure, or assure?
early 13c., from Old French flater "to flatter" (13c.), originally "stroke with the hand, caress," from Frankish *flat "palm, flat of the hand" (see flat (adj.)). "[O]ne of many imitative verbs beginning with fl- and denoting unsteady or light, repeated movement" [Liberman]. Related: Flattered; flattering.