Impugn :: Impugn (v. t.) To attack by words or arguments; to contradict; to assail; to call in question; to make insinuations against; to gainsay; to oppose.
Impugnable :: Impugnable (a.) Capable of being impugned; that may be gainsaid.
Impugnation :: Impugnation (n.) Act of impugning; opposition; attack.