To bad-mouth someone is to say unkind things about them, usually when they are not in the room. You are tearing down their name — a person, a shop, a film, a plan. It is informal spoken English, a little sharp, and it almost always sounds like gossip or an attack, not like a careful review. You can bad-mouth a product (“Don’t buy that phone”) or a person. Though actually, in writing you will also see it stuck together as badmouth, and the -ing form bad-mouthing / badmouthing; the meaning stays the same. It is stronger than “I don’t like it” and ruder than “I disagree.”

說壞話、講人壞話