The Collaborative International Dictionary of English v.0.48:

Rebut \Re*but"\ (r[-e]*b[u^]t"), v. t. [imp. & p. p. Rebutted;
   p. pr. & vb. n. Rebutting.] [OF. rebouter to repulse, drive
   back; pref. re- + bouter to push, thrust. See 1st Butt,
   Boutade.]
   [1913 Webster]
   1. To drive or beat back; to repulse.
      [1913 Webster]

            Who him, rencount'ring fierce, as hawk in flight,
            Perforce rebutted back.               --Spenser.
      [1913 Webster]

   2. (Law) To contradict, meet, or oppose by argument, plea, or
      countervailing proof. --Abbott.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rebut \Re*but"\, v. i.
   1. To retire; to recoil. [Obs.] --Spenser.
      [1913 Webster]

   2. (Law) To make, or put in, an answer, as to a plaintiff's
      surrejoinder.
      [1913 Webster]

            The plaintiff may answer the rejoinder by a
            surrejoinder; on which the defendant may rebut.
                                                  --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rebut
    v 1: overthrow by argument, evidence, or proof; "The speaker
         refuted his opponent's arguments" [syn: refute, rebut]
    2: prove to be false or incorrect [syn: refute, rebut,
       controvert]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "rebut":
   advocate, allege in support, answer, answer conclusively,
   argue down, argue for, assert, be contrary to, belie,
   blow sky-high, break, call into question, challenge, champion,
   confound, confute, contend for, contest, contradict, contravene,
   controvert, counter, countercharge, counterclaim, cross, crush,
   defeat, defend, demolish, deny, destroy, devastate, disconfirm,
   discredit, dismiss, dispose of, disprove, espouse, evert, expose,
   finish, floor, gainsay, hold off, invalidate, keep off, maintain,
   make a plea, make a rebuttal, negate, nonplus, oppugn, overthrow,
   overturn, overwhelm, parry, plead for, puncture, put to silence,
   rebuff, reduce to silence, refute, reject, rejoin, repel, reply,
   repulse, respond, riposte, ruin, say in defense, settle,
   shoot down, shut up, silence, smash all opposition, speak for,
   speak up for, squash, squelch, stand up for, stave off,
   stick up for, subvert, support, surrebut, surrejoin, sustain,
   traverse, undermine, uphold, upset, urge reasons for, ward off