Bad-mouth someone说某人的坏话
bad-mouth sb说某人的坏话
I hate to bad-mouth people我讨厌说别人坏话的人
V-T（及物动词） If someone bad-mouths you, they say unpleasant things about you, especially when you are not there to defend yourself. (尤指在他人背后)诽谤
Both men continually bad-mouthed each other.
两个男人继续彼此诽谤。