Is it ensure, insure, or assure?
early 14c., "to train or instruct in some specific subject," from Old French informer "instruct, inform, teach," and directly from Latin informare "to shape, form," figuratively "train, instruct, educate," from in- "into" (see in- (2)) + formare "to form, shape," from forma "form" (see form (n.)). Varied with enform until c.1600. Sense of "report facts or news" first recorded late 14c. Related: Informed; informing.