Gainsay
/ˌɡeɪnˈseɪ/
Definition 1 of 2
Pro Tip 1/3Strong Disagreement
Gainsay expresses a firm and strong disagreement with someone or something. 
She would gainsay any suggestion that was against her principles.
Pro Tip 2/3Intensity
Using 'gainsay' adds intensity to the act of disputing a statement. 
His theories were difficult to gainsay because of the extensive evidence.
Pro Tip 3/3Literary Flair
The word 'gainsay' has a literary quality and might be more common in written texts. 
In his novels, the protagonist would often gainsay societal norms.