Blow the gaff
This phrase would be found in British films of a certain vintage, generally with a criminal flavour, and is the equivalent of spilling the beans, giving away a secret. The key to understanding the phrase lies in the word gaff.
The earliest usage of the word gaff in the English language dates back to the early 14th century where it was used to describe a hooked stick or barbed spear which would be deployed to land fish and which itself owed its origin to the Provencal word, gaf, meaning boat hook. The French used the word figuratively to mean an error or blunder – what we may term a gaffe. Quite why is unclear but perhaps the realisation that you have made an error is akin, at least to someone with a vivid imagination, to being stunned on the head with a stick.
Another usage of the word gaff is to denote a place, particularly one in which you live or one which dispenses refreshments. I find I use it in this context quite regularly. It is likely that etymologically this usage comes from the Romany word gav which was used to denote a town or a market town, the latter being of particular interest to the itinerant traders. In 1753 gaff was used to describe a fair but by 1812 had gone decidedly down market when it was defined as “a cheap music hall or theatre; place of amusement for the lowest classes”. But it is hard to see how this meaning could have influenced our phrase.
I think more interesting are two other usages of the word gaff. In Vocabulum or The Rogue’s Lexicon, George Washington Matsell defined gaff as “a ring worn on the fore-finger of the dealer. It has a sharp point on the inner side, and the gambler, when dealing from a two-card box, can deal out the card he chooses”. Also, interestingly, gaff in certain English dialects meant loud and coarse talk and in the Scottish dialect gaff meant to talk loudly and merrily.
One of the usages of blow was to mean to inform on – Daniel Defoe used it thus in History of Colonel Jack (1723), “for I am blown and they will betray me” but the earliest usage was in association with gab which itself meant talk or conversation rather than gaff. The Oath of the Canting Crew which Robert Goadby preserved for posterity in 1749 ran, “I, Crank Cuffin, swear to be/ true to this fraternity;/ that I will in all obey/ Rule and order of the lay./ Never blow the gab or squeak;/ never snitch to bum or beak”. By 1812, however, the phrase had become blow the gaff as James Hardy Vaux helpfully explained in A Vocabulary of the Flash Language, “A person having any secret in his possession , or a knowledge of anything injurious to another, when at east 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”.
Quite how gaff replaced gab is unclear. The most rational explanation might be that the gaff in our phrase is the card sharp’s ring and blowing the gaff is exposing him as a cheat. Alluring as this explanation may be there is the little difficulty of dates and location – gaff is American slang defined in 1859 whereas blowing the gaff was in circulation in 1812 at least. These are not insurmountable difficulties without having to resort to Carl Sagan’s the absence of evidence is not evidence of absence but the more likely explanation is that as gab and gaff were both nouns for talk, they were interchangeable in our expression.