A person who talks too much, reveals secrets, or cannot keep information private.
Compound of 'big' and 'mouth,' literally describing someone with a large or overly active mouth. American slang from the early 20th century.
The insult 'bigmouth' has been around for over a century because gossiping and spreading secrets is a fundamentally human social problem—which is probably why it still stings today when someone calls you out for not keeping quiet.
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