This expression means to reveal a secret. 'Gaff' is a term for some hidden trick used as a cheating device in gambling. Originally this was a small hook set in a ring that was used by card-sharps to grip the cards, so the origin is probably in the hook sense of 'gaff'. The verb 'to blow' here has an informal meaning of informing on, betraying or exposing someone. The saying would at first have meant exposing the trade secrets of gamblers and cheats.

Übersetzung:

etwas ausplaudern

Gegenteil(e)

The opposite of "to blow the gaff" is "to keep a secret".

Synonym(e):

You could also say: to reveal a secret.