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1. verb To speak negatively or unflatteringly about someone or something. Why are you always bad-mouthing Tiffany? You don't even know her that well!
2. noun One who says negative or unflattering things about someone or something. He's a real bad-mouth, so don't listen to anything he says about me!
bad-mouth someone or something
to say negative things about someone or something. Mr. Smith was always bad-mouthing Mrs. Smith. They just didn't get along. John bad-mouths his car constantly because it is too small for him.
Disparage or criticize unduly, malign, as in Why do you constantly bad mouth your colleagues? This term is believed to be of African origin, where the phrase bad mouth signifies a curse or evil spell. [1930s]