"to have illusions," 1650s, 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 "wander in mind, be at a loss, be beside oneself (with grief, joy, perplexity), be distraught," also "wander about," which probably is related to alaomai "wander about" [Barnhart, Klein]. The Latin ending probably was influenced by vaticinari "to prophecy," also "to rave." Older in English in a rare and now obsolete transitive sense "deceive" (c. 1600); occasionally used 19c. in transitive sense "to cause hallucination." Related: Hallucinated; hallucinating.