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" (from PIE root *po(i)- "to drink").
For form evolution from Latin to French, compare 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 virus. The Old English word was ator (see attercop) or lybb (cognate with Old Norse lyf "medicinal herbs;" see leaf (n.)). Slang sense of "alcoholic drink" first attested 1805, American English.
For sense evolution, compare Old French enerber, enherber "to kill with poisonous plants." In many Germanic languages "poison" is named by a word equivalent to English gift (such as Old High German gift, German Gift, Danish and Swedish gift; Dutch gift, vergift). This shift might have been partly euphemistic, partly by influence of Greek dosis "a portion prescribed," literally "a giving," used by Galen and other Greek physicians to mean an amount of medicine (see dose (n.)).
Figuratively from late 15c.; of persons by 1910. As an adjective from 1520s; with plant names from 18c. Poison ivy first recorded 1784; poison oak is from 1743. Poison gas first recorded 1915. Poison-pen (letter) popularized 1913 by a notorious criminal case in Pennsylvania, U.S.; the phrase dates to 1898.