Is it ensure, insure, or assure?
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.
antagonist an·tag·o·nist (ān-tāg'ə-nĭst)
Something, such as a muscle, disease, or physiological process, that neutralizes or impedes the action or effect of another.