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]