tv. to speak ill of someone or something. (See also dirty mouth; poor-mouth.) : I wish you would stop bad-mouthing my car.
n. someone who speaks ill of someone or something. : Harry is such a bad-mouth!
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw-Hill Education.
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Idioms & Phrases
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]