disparage
To talk about someone in a mean, put-down way.
To disparage means to speak about someone or something in a way that makes them seem unimportant, worthless, or inferior. When you disparage something, you put it down, belittle it, or treat it with contempt, going beyond fair disagreement or constructive criticism.
Imagine a student who worked hard on a science project. If a classmate says, “I think you could have explained the experiment more clearly,” that's constructive criticism. But if they say, “This project is stupid, and anyone could have done better,” they're disparaging the work. They're trying to make it seem worthless rather than offering helpful feedback.
People might disparage ideas they disagree with, accomplishments they're jealous of, or things they simply don't understand. A coach who constantly disparages players instead of helping them improve is doing real harm. Someone who disparages books they haven't read or subjects they've never studied reveals more about their own character than about what they're criticizing.
When you hear someone disparaging another person's efforts or achievements, notice whether they're offering genuine insight or just tearing something down to make themselves feel superior.