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Hi

Would you say that the red word is common in everyday conversation? Any suitable/formal synonym?

Don't be such a windbag.

She is a terrible windbag. She could bore you to death.

Thanks,

Tom

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Not common, no, but not rare either. It's OK to use in everyday speech (at least in BrE).

I can't think of a formal one-word synonym.

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American English:

chatterbox (for younger people), windbag (for older people)

These are negative:

blabbermouth (one who gossips or gives away confidential information), tattletale, (a child who tells people about the bad things their friend did).