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.]
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            His unkindness may defeat my life.    --Shak.
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   2. To render null and void, as a title; to frustrate, as
      hope; to deprive, as of an estate.
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            He finds himself naturally to dread a superior Being
            that can defeat all his designs, and disappoint all
            his hopes.                            --Tillotson.
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            The escheators . . . defeated the right heir of his
            succession.                           --Hallam.
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            In one instance he defeated his own purpose. --A. W.
                                                  Ward.
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   3. To overcome or vanquish, as an army; to check, disperse,
      or ruin by victory; to overthrow.
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   4. To resist with success; as, to defeat an assault.
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            Sharp reasons to defeat the law.      --Shak.

   Syn: To baffle; disappoint; frustrate.
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WordNet (r) 3.0 (2006):

defeated
    adj 1: beaten or overcome; not victorious; "the defeated enemy"
           [ant: undefeated]
    2: disappointingly unsuccessful; "disappointed expectations and
       thwarted ambitions"; "their foiled attempt to capture
       Calais"; "many frustrated poets end as pipe-smoking
       teachers"; "his best efforts were thwarted" [syn: defeated,
       disappointed, discomfited, foiled, frustrated,
       thwarted]
    n 1: people who are defeated; "the Romans had no pity for the
         defeated" [syn: defeated, discomfited]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "defeated":
   all up with, baffled, balked, beat, beaten, bested, betrayed,
   bilked, blasted, blighted, chapfallen, confounded, crestfallen,
   crossed, crushed, dashed, disappointed, discomfited, dished,
   disillusioned, dissatisfied, done for, done in, down, fallen,
   fixed, floored, foiled, frustrated, hors de combat, ill done-by,
   ill-served, lambasted, lathered, let down, licked, on the skids,
   out of countenance, outdone, overborne, overcome, overmastered,
   overmatched, overpowered, overridden, overthrown, overturned,
   overwhelmed, panicked, put to rout, regretful, routed, ruined,
   scattered, settled, silenced, skinned, skinned alive,
   sorely disappointed, soured, stampeded, thwarted, trimmed,
   trounced, undone, upset, whelmed, whipped, worsted