c.1200, "a deadly potion or substance," also figuratively, from Old French poison, puison (12c., Modern French poison) "a drink," especially a medical drink, later "a (magic) potion, poisonous drink" (14c.), from Latin potionem (nominative potio) "a drinking, a drink," also "poisonous drink" (Cicero), from potare "to drink" (see potion).
For form evolution from Latin to French, cf. raison from rationem. The Latin word also is the source of Old Spanish pozon, Italian pozione, Spanish pocion. The more usual Indo-European word for this is represented in English by