opponent (n.)

"one who opposes, an adversary, an antagonist," 1580s, from noun use of Latin opponentem (nominative opponens), present participle of opponere "oppose, object to," literally "set against, set opposite," from assimilated form of ob "in front of, in the way of" (see ob-) + ponere "to put, set, place" (see position (n.)). Originally "one who maintains a contrary argument in a disputation;" the general sense is by 1610s.