Gainsay

gainsay [gān-sā]
v. to deny; contradict; speak or act against

[Derives from Old English gegn ("against") + say.]

The Old English gegn is also the root of again, which normally now means "one more time," but can also mean "in the opposite direction." In that latter usage, it's easier to see the connection between again and against.

There used to be a handful of other words in English which had gain- as their prefix. Save gainsay, the others all disappeared from our lexicon. These included gain-taking ("taking back again"), gainclap ("a counterstroke"), gainbuy ("redeem") and gainstand ("to oppose"). Gain ("profit; increase") and gainful ("profitable") have an unrelated root that comes from Old French gaaing ("to till; earn; win"). The Spanish verb ganar ("to win; earn; gain") has the same origins.

In Theodore Rex, Edmund Morris quoted Teddy Roosevelt using gainsay to mean contradict:

Facts, which I cannot gainsay, force me to believe that the conservation of our national resources is the most weighty question now before the people of the United States.