Is it ensure, insure, or assure?
c.1600, "deceive," from Latin alucinatus, later hallucinatus, past participle of alucinari "wander (in the mind), dream; talk unreasonably, ramble in thought," probably from Greek alyein, Attic halyein "be distraught," probably related to alaomai "wander about" [Barnhart, Klein]. The Latin ending probably was influenced by vaticinari "to prophecy," also "to rave." Sense of "to have illusions" is from 1650s. Occasionally used 19c. in transitive senses, "to cause hallucination." Related: Hallucinated; hallucinating.