Gainsay (verb)

1. To deny or contradict (something said or done by someone else); to speak against.
2. To oppose or resist (an action or proposal); to speak against.
3. To challenge the validity of (a statement or claim).

Origin:

From old english geǣnsæġian, from geǣn ("against") + sæġian ("to say"), equivalent to gain- +‎ say.

Examples:

1. He gainsaid the prosecutor's claim that the accused was guilty.
2. She gainsaid the company's decision to lay off employees.
3. He gainsaid the statement made by the witness, claiming it was untrue.
4. The company's CEO gainsaid the accusations that the company was responsible for the pollution.
5. The lawyer gainsaid the evidence presented by the prosecution.