Meaning:
To make public a secret or reveal a plot.
Background:
When someone is said to blow the gaff it is usually when exposing some crime
or underworld activity. It's the kind of expression you might expect to hear
James Cagney use in some mobster film.
...
The first printed example that I know of is from 1819, in James Vaux's
A Vocabulary of the Flash Language, the first dictionary ever compiled in
Australia from The Memoirs of James Hardy Vaux, 1819.
BLOW THE GAFF, a person having any secret in his possession, or a knowledge
of any thing injurious to another, when at last induced from revenge, or
other motive, to tell it openly to the world and expose him publicly, is
then said to have blown the gaff upon him.
Vaux was British by birth and was well acquainted with the language of the
criminal classes. He was an inveterate thief and swindler and was transported
to Australia as a convict in 1801.
- www.phrases.org.uk [edited]
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The end of the third week of June, Bill took the east-bound local train to the
port city to pass the College English Test level 4, a pre-requisite for college
graduation, for his best bud.
That muggy overcast morning, he showed up taut, wooden, and on an empty stomach
(He always skipped breakfast on a test day) at the classroom entrance where the
three dozens of exam sitters were verified. A first-time ringer, he didn't try
hard to relax. They were supposed to fret and the last thing he wanted was to
stand out like a sore thumb by appearing too tense or lax. His palms started to
sweat as he handed in his student ID booklet and his heart missed a beat or two
as the rheumy eyes of the 50-ish woman behind a pair of rimless glasses darted
from his face to his mugshot and back. It was not until they started working on
the problems before he could ease up.
In the next two hours had the proctors taken a closer look at the partial
embossed chop over the right bottom corner of his photo, they would've noticed
its poor quality. They might have suspected, double-checked, blown the gaff on
the spot, and both Bill and his chum would've been expelled from uni.
