Definitions:

1. To deny, contradict, or speak against something. VERB
   • No one could gainsay her success after she won the award.
2. Used in formal or literary contexts, where someone might challenge or dispute a statement or an assertion. VERB
   • The politician's charisma was undeniable, and his opponents could not gainsay his popularity among voters.

Synonyms:

• deny
• dispute
• refute
• reject