gain·say (g?n-s??, g?n?s??)tr.v. gain·said (-s?d?, -s?d?), gain·say·ing, gain·says (-s?z?, -s?z?) 1. To declare to be false; deny. See Synonyms at deny.2. To oppose (someone), especially by contradiction: “She was going to fashion the end of her existence in her own way, and in this determination she would not be gainsaid” (Louis Auchincloss).[Middle English gainsayen : gain-, against (from Old English gegn-) + sayen, to say (from Old English secgan; see say).]gain·say?er n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.