Is it ensure, insure, or assure?
late 14c., "to hold, occupy, reside in" (without regard to ownership), a back formation from possession and in part from Old French possesser "to have and hold, take, be in possession of" (mid-13c.), from Latin possess-, past participle stem of possidere "to have and hold, possess, be master of, own," from posse "to be able," from potis "able, powerful" (see potent) + esse "to be" (see be). Meaning "to hold as property" is recorded from c.1500. Demonic sense is recorded from 1530s (implied in possessed). Related: Possessed; possessing.