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]