Badmouth (verb)

To speak negatively or critically about someone or something.

Origin:

From "bad" and "mouth" meaning to speak negatively or critically about someone or something.

Examples:

1. He had a tendency to badmouth his colleagues behind their backs.
2. She badmouthed her ex-boyfriend to all her friends.
3. He badmouthed the company after he got fired.
4. She badmouthed the new product before it was even released.
5. He badmouthed the new manager to the whole department.