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.
I will give you a mouth and wisdom which all your
adversaries shall not be able to gainsay nor resist.
The just gods gainsay
That any drop thou borrow'dst from thy mother,
My sacred aunt, should by my mortal sword
Be drained. --Shak.