Yes. "To badmouth" somebody is to say bad things about them, usually behind their back (although not always). It's a slang (colloquial) expression.
Maria is always badmouthing me.
Jake badmouths his brother all the time.
Christina was badmouthing Maria all evening.
One thing: I've never heard it used in the past simple tense (Jake badmouthed his brother last night). The phrase always seems to be used to describe a continuous action.
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