blowing the gaff

Verb

blow the gaff (third-person singular simple present blows the gaff, present participle blowing the gaff, simple past blew the gaff, past participle blown the gaff)

1. (UK, obsolete, slang) To disclose a secret.
   • But, my child, if Geoffrey is to be got out at this immature stage it blows the gaff completely. Half of the crowd will vanish, including all the worst, and we shall have no proof at all of the real depths of the conspiracy.1934, Ernest Bramah, The Bravo of London

   Synonyms:

   let the cat out of the bag

2. (UK, obsolete, slang) To inform against a person.