climbing plant, Old English ifig, from West Germanic *ibakhs (source also of Middle Low German iflof, Dutch eiloof, Old High German ebahewi, German Efeu), a word of unknown origin; the second element in the Old High German word might be heu "hay."
Ivy bush as a sign of a tavern where wine is served is attested from mid-15c. (the ivy being sacred to Bacchus). Ivy League, inspired by the image of old, ivy-mantled walls, dates to 1935, originally in reference to a conference of football teams agreeing to organize teams and play games by set rules; it consists of Brown, Columbia, Cornell, Dartmouth, Harvard, Pennsylvania, Princeton, and Yale.
Incidentally, "Ivy League" is a poor name; it suggests that athletic morality is concentrated in a few very old colleges. Obviously, membership should be determined not by the amount of ivy on an institution's walls, but by its willingness to adopt and follow certain fundamental principles of education and sport. [Princeton Alumni Weekly, Dec. 6, 1935]
"seizing or grasping, adapted for taking and holding," 1771, from French préhensile "adapted for grasping" (Buffon), from Latin prehensus, past participle of prehendere "to grasp, seize, get hold of," from prae- "before" (see pre-) + -hendere, from PIE root *ghend- "to seize, take."
Latin -hendere perhaps is related to hedera "ivy," via the notion of "clinging." De Vaan writes, "Of course, ivy is a climbing (or ground-creeping) plant, and one may surmise that its name means 'the grabbing one', but this is just a guess, especially since the morphology is uncommon: no s-stem of this root is attested elsewhere in IE."
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.