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1.
To let out information that should have been secret.
We just dumped the waste chemicals into the reservoir until we sacked Eddie and he had to bloody go and blow the gaff!
by pidulgi August 20, 2013
 
2.
To knock one out on to a piece of cloth - in most cases a hankerchief - roll it up and hide it under the bed covers.
Guy: hey, why you late today? Been waiting for ages...

Friend: Sorry man, had to blow the gaff first.

Guy: Did you wash your hands?
by Crazy Meka September 16, 2009