Publicly bad-mouthing someone is an act of defaming them, which is defined as making false and malicious statements about someone in order to damage their reputation. This term is often used to describe a situation where someone speaks negatively about another person in a public setting, such as on social media or in a public forum. Defaming someone can have serious consequences, including legal action, so it is important to be aware of the potential consequences before engaging in this type of behavior.