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.
See also: blow, gaff

blow the gaff

  (British old-fashioned, informal)
to cause trouble for someone by letting other people know something that they were trying to keep secret (often + on ) They killed Green because he was about to blow the gaff on their drug dealing.
See also: blow, gaff