The Collaborative International Dictionary of English v.0.48:

Defeat \De*feat"\, v. t. [imp. & p. p. Defeated; p. pr. & vb.
   n. Defeating.] [From F. d['e]fait, OF. desfait, p. p. ofe
   d['e]faire, OF. desfaire, to undo; L. dis- + facere to do.
   See Feat, Fact, and cf. Disfashion.]
   1. To undo; to disfigure; to destroy. [Obs.]
      [1913 Webster]

            His unkindness may defeat my life.    --Shak.
      [1913 Webster]

   2. To render null and void, as a title; to frustrate, as
      hope; to deprive, as of an estate.
      [1913 Webster]

            He finds himself naturally to dread a superior Being
            that can defeat all his designs, and disappoint all
            his hopes.                            --Tillotson.
      [1913 Webster]

            The escheators . . . defeated the right heir of his
            succession.                           --Hallam.
      [1913 Webster]

            In one instance he defeated his own purpose. --A. W.
                                                  Ward.
      [1913 Webster]

   3. To overcome or vanquish, as an army; to check, disperse,
      or ruin by victory; to overthrow.
      [1913 Webster]

   4. To resist with success; as, to defeat an assault.
      [1913 Webster]

            Sharp reasons to defeat the law.      --Shak.

   Syn: To baffle; disappoint; frustrate.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "defeating":
   ascendant, baffling, confounding, conquering, disconcerting,
   dominant, flushed with success, frustrating, in ascendancy,
   in the ascendant, overcoming, prevailing, successful, triumphal,
   triumphant, vanquishing, victorious, winning