adversary (n.)

"unfriendly opponent, enemy" (originally especially of Satan as the enemy of mankind), mid-14c., aduersere, from Anglo-French adverser (13c.), Old French adversarie (12c., Modern French adversaire) "hostile opponent, enemy," or directly from Latin adversarius "an opponent, rival, enemy," noun use of adjective meaning "opposite, hostile, contrary," literally "turned toward one," from adversus "turned against, turned toward, fronting, facing," figuratively "hostile, adverse, unfavorable," past participle of advertere "to turn toward," from ad "to" (see ad-) + vertere "to turn, turn back, be turned; convert, transform, translate; be changed" (see versus). The Latin word is glossed in Old English by wiðerbroca.