1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs