blow the gaff

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blow the gaff

slang To talk about or reveal a private or secret matter. Primarily heard in UK. 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

blow the gaff

reveal or let out a plot or secret. British informal
The word gaff is recorded from the early 19th century, but its origins are uncertain.
See also: blow, gaff

blow the ˈgaff (on somebody/something)

(British English, informal) reveal a secret: She didn’t want anyone to know where she had been, but her husband blew the gaff. OPPOSITE: keep mum
See also: blow, gaff