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"abuse someone verbally," 1941, probably ultimately from noun phrase bad mouth (1835), in Black English, "a curse, spell," translating an idiom found in African and West Indian languages. Related: Bad-mouthed; bad-mouthing.
: If you can't say anything good, at least don't be a bad mouthverb phrase
To disparage; denigrate: He bad-mouthed everybody (1930s+)