To give away information, reveal a secret, inform on someone. This picturesque phrase was derived from the archaic term 'gaff' meaning a trickster's strategy or paraphernalia. Although it dates from the early 19th century and often evokes the world of spivs or gangsters, the expression is still used. Confusingly, blow the gaff could conceivably also now mean 'leave (blow) the premises (gaff)'.