Opponent
late 16th century (denoting a person opening an academic debate by proposing objections to a philosophical or religious thesis): from Latin opponent- ‘setting against’, from the verb opponere, from ob- ‘against’ + ponere ‘place’.
wiktionary
Borrowed from Latin oppōnēns(“opposing”), present active participle of oppōnō(“I oppose”).
etymonline
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.
