blow the gaff

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blow the gaff

slang To talk about or reveal a private or secret matter. I can't believe you blew the gaff and talked about the plans for her surprise party right to her face! Don't tell your little brother anything you want to keep secret because he'll definitely blow the gaff.
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blow the gaff

BRITISH, INFORMAL
If you blow the gaff, you tell people something which was supposed to be kept secret. He is certainly not the first minister to blow the gaff. Note: `Blow' here means `reveal'. In the 19th century, `gaff' was a slang word used to refer to dishonest behaviour which was intended to deceive people.
See also: blow, gaff