Slang. to speak critically and often disloyally of; disparage: Why do you bad-mouth your family so much?
Also, badmouth.
Origin: 1935–40; orig. a curse, spell (the sense recorded in Gullah); cf. Vai (Mande language of Liberia and Sierra Leone) dà nyà mà curse, lit., bad mouth
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.
Cite This Source
Word Origin & History
bad-mouth (v.)
"abuse someone verbally," 1941, probably ultimately from noun phrase bad mouth (1835), in black Eng., "a curse, spell," translating an idiom found in African and West Indian languages.
[bad-mouth or, sometimes, -mouth] 
