To speak badly about someone or criticize them when they are not present; to say mean things behind someone's back.
From Old English 'bæc' (back) + 'bitan' (to bite). This word originated in Middle English around the 13th century as a metaphor combining the idea of attacking from behind with biting, capturing the act of verbal treachery in vivid image.
Every culture has a word for this because backbiting is a universal human temptation—the elegance of 'backbite' is that it captures the sneakiness and violence of gossip in one perfect metaphor that's been used for over 800 years.
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