c. 1200, poisoun, "a deadly potion or substance," also figuratively, "spiritually corrupting ideas; evil intentions," 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").
A doublet of potion. For similar form evolution from Latin to French, compare raison from rationem, trahison from traditionem. 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.)).
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.)).
Of persons detested or regarded as exerting baleful influence, by 1910. The slang meaning "alcoholic drink" is by 1805 in American English (potus as a past-participle adjective in Latin meant "drunken").
As an adjective from 1520s; with plant names from 18c. Poison ivy is recorded by 1784 for a shrub-vine of North America causing an itching rash on contact; poison oak for poison ivy or related species is by 1743. Poison sumac (1817), causing an even more severe rash, is a swamp-border tree noted for the brilliant red of its leaves in fall. Poison gas is recorded from 1915. Poison-pen (letter) was popularized 1913 by a notorious criminal case in Pennsylvania, U.S.; the phrase dates to 1898.