A blabbermouth is a person who cannot keep a secret because they talk too much. The picture in the word is simple: a mouth that blabbers. You call someone a blabbermouth after they have already told the class about the surprise, the crush, or the plan that was meant to stay quiet. It is informal and teasing, sometimes affectionate (“you old blabbermouth”), sometimes sharp. It is not a formal word, and it is not as heavy as snitch or traitor — a blabbermouth is usually careless, not plotting. Though actually, people also stretch it to mean anyone who never stops talking, even when no secret is involved. The core idea stays the same: too much talk, and private things do not stay private.

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