The Collaborative International Dictionary of English v.0.48:

Gainsay \Gain`say"\ (? or ?; 277), v. t. [imp. & p. p.
   Gainsaid (? or ?); p. pr. & vb. n. Gainsaying.] [OE.
   geinseien, ageinseien. See Again, and Say to utter.]
   To contradict; to deny; to controvert; to dispute; to forbid.
   [1913 Webster]

         I will give you a mouth and wisdom which all your
         adversaries shall not be able to gainsay nor resist.
                                                  --Luke xxi.
                                                  15.
   [1913 Webster]

         The just gods gainsay
         That any drop thou borrow'dst from thy mother,
         My sacred aunt, should by my mortal sword
         Be drained.                              --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gainsay
    v 1: take exception to; "She challenged his claims" [syn:
         challenge, dispute, gainsay]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "gainsay":
   abjure, assert the contrary, be contrary to, belie,
   call into question, challenge, combat, contest, contradict,
   contravene, controvert, counter, cross, deny, disaffirm, disallow,
   disavow, disclaim, disown, disprove, dispute, fight, forswear,
   impugn, join issue upon, negate, negative, not accept, not admit,
   nullify, oppose, oppugn, rebut, recant, refuse to admit, refute,
   reject, renounce, repudiate, resist, retract, revoke, take back,
   take issue with, traverse, withstand