antagonist n.

word-forming element of Greek origin meaning "against, opposed to, opposite of, instead," shortened to ant- before vowels and -h-, from Old French anti- and directly from Latin anti-, from Greek anti (prep.) "over, against, opposite; instead, in the place of; as good as; at the price of; for the sake of; compared with; in opposition to; in return; counter-," from PIE *anti "against," also "in front of, before" (from root *ant- "front, forehead," with derivatives meaning "in front of, before"), which became anti- in Italian (hence antipasto) and French.

It is cognate with Sanskrit anti "over, against," and Old English and- (the first element in answer). A common compounding element in Greek, in some combinations it became anth- for euphonic reasons. It appears in some words in Middle English but was not commonly used in English word formations until modern times. In a few English words (anticipate, antique) it represents Latin ante.

In noun compounds where it has the sense of "opposed to, opposite" (Antichrist, anti-communist) the accent remains on the anti-; in adjectives where it retains its old prepositional sense "against, opposed to," the accent remains on the other element (anti-Christian, anti-slavery).

late 14c., "mental suffering" (especially that of Christ in the Garden of Gethsemane), from Old French agonie, agoine "anguish, terror, death agony" (14c.), and directly from Late Latin agonia, from Greek agōnia "a struggle for victory" (in wrestling, etc.), in a general sense "exercise, gymnastics;" also of mental struggles, "agony, anguish." This is from agōn "assembly, mass of people brought together," especially to watch the games, hence, "a contest," then, generally, "any struggle or trial;" from the verb agein "put in motion, move" (here specifically as "assemble, bring together"), from PIE root *ag- "to drive, draw out or forth, move."

Specifically of the struggle that precedes natural death (mortal agony) from 1540s. The sense development perhaps involves "pain so severe as to cause struggling." Sense of "extreme bodily suffering" is recorded by c. 1600.