Translation

否定する / 反論する / 異議を唱える

Meaning

Gainsay means to deny, dispute, or contradict a statement. It is old-fashioned and most often appears in the phrase cannot be gainsaid, meaning cannot reasonably be denied.

Examples

• その証拠の重要性は否定できません。
  The importance of the evidence cannot be gainsaid.
• 誰も彼女の勇気を否定しようとはしませんでした。
  No one tried to gainsay her courage.