Quiz: Remember the definition of mal de mer?
"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+)