antagonist: [16] Greek agón (source of English agony) meant ‘contest, conflict’. Hence the concept of ‘struggling against (anti-) someone’ was conveyed in Greek by the verb antagōnízesthai. The derived noun antagōnistés entered English via French or late Latin. => agony
antagonist (n.)
1590s, from French antagoniste (16c.) or directly from Late Latin antagonista, from Greek antagonistes "competitor, opponent, rival," agent noun from antagonizesthai "to struggle against, oppose, be a rival," from anti- "against" (see anti-) + agonizesthai "to contend for a prize," from agon "contest" (see agony). Originally in battle or sport, extended 1620s to any sphere of human activity.