The old-fashioned term “gainsay” has ancient roots going back to Old English, in which “gęgn-” and “géan” both implied reversal or opposition. Adding “gain-” as a prefix to “say” means “to say in opposition,” but it’s most often expressed in the negative as a form of confirmation. For example, an argument “cannot be gainsaid,” or “no one may gainsay” a well-supported opinion. In both cases, the inability to gainsay is confirmation of the validity of the argument or fact.