The Collaborative International Dictionary of English v.0.48:

Contradict \Con`tra*dict\, v. i.
   To oppose in words; to gainsay; to deny, or assert the
   contrary of, something.
   [1913 Webster]

         They . . . spake against those things which were spoken
         by Paul, contradicting and blaspheming.  --Acts xiii.
                                                  45.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Contradict \Con`tra*dict"\, v. t. [imp. & p. p. Contradicted;
   p. pr. & vb. n. Contradicting.] [L. contradictus, p. p. of
   contradicere to speak against; contra + dicere to speak. See
   Diction.]
   1. To assert the contrary of; to oppose in words; to take
      issue with; to gainsay; to deny the truth of, as of a
      statement or a speaker; to impugn.
      [1913 Webster]

            Dear Duff, I prithee, contradict thyself,
            And say it is not so.                 --Shak.
      [1913 Webster]

            The future can not contradict the past.
                                                  --Wordsworth.
      [1913 Webster]

   2. To be contrary to; to oppose; to resist. [Obs.]
      [1913 Webster]

            No truth can contradict another truth. --Hooker.
      [1913 Webster]

            A greater power than we can contradict
            Hath thwarted our intents.            --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

contradict
    v 1: be in contradiction with [syn: contradict, belie,
         negate]
    2: deny the truth of [syn: contradict, negate, contravene]
    3: be resistant to; "The board opposed his motion" [syn:
       oppose, controvert, contradict]
    4: prove negative; show to be false [syn: negate,
       contradict] [ant: affirm, confirm, corroborate,
       substantiate, support, sustain]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "contradict":
   abjure, abrogate, agree to disagree, annul, answer,
   answer conclusively, antagonize, argue down, assert the contrary,
   be antipathetic, be at cross-purposes, be contrary to, be inimical,
   be opposed to, beat against, belie, break, break off, brush aside,
   call into question, challenge, chuck, chuck out, clash, collide,
   conflict, conflict with, confound, confute, contemn, contest,
   contrapose, contrast with, contravene, controvert, counter,
   counteract, counterattack, counterbalance, countercheck,
   counterpoise, counterpose, countervail, counterwork, cross, crush,
   decline, defeat, demolish, deny, despise, differ, disaccord,
   disaffirm, disagree, disallow, disapprove, disavow, discard,
   disclaim, discount, disdain, dismiss, disown, dispose of, disprove,
   dispute, disregard, dissent, except, exclude, falsify, finish,
   floor, forbid, forswear, gainsay, garble, go against,
   go counter to, ignore, impugn, interfere with, jangle, jar,
   join issue upon, jostle, juxtapose in opposition, lock horns,
   meet head-on, militate against, mismatch, mismate, negate,
   negative, nonplus, not accept, not admit, nullify, object, offset,
   oppose, oppugn, overthrow, overturn, overwhelm, parry, pass by,
   pass up, play at cross-purposes, push aside, put to silence,
   rebuff, rebut, recant, reduce to silence, refuse, refuse to admit,
   refuse to consider, refute, reject, renounce, repel, repudiate,
   repulse, resist, retract, reverse, revoke, run against,
   run counter to, scout, set off, settle, shove away, shut up,
   silence, smash all opposition, spurn, squash, squelch, subvert,
   swim upstream, take back, take issue with, throw away, throw out,
   traverse, turn away, turn out, undermine, upset, vary, waive,
   work against