gainsay (v.) Look up gainsay at Dictionary.com
"contradict," c.1300, literally "say against," from Old English gegn- "against" (see again) + say (v.). "Solitary survival of a once common prefix" [Weekley], which was used to form such now-obsolete compounds as gain-taking "taking back again," gainclap "a counterstroke," gainbuy "redeem," and gainstand "to oppose." Related: Gainsaid; gainsaying.