Present participle of badmouth; the act of speaking negatively or unfairly about someone.
From 'badmouth' (itself 'bad' + 'mouth') + '-ing' (present participle). The -ing form emphasizes the ongoing or habitual nature of the insulting behavior.
Badmouthing is technically slander or defamation if done publicly and falsely—but the casual word 'badmouthing' lets us gossip guilt-free, which might be why we have this word instead of just saying 'insulting' or 'slandering'!
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