The College cove has numbered him, and if he is knocked down he’ll be twisted; the turnkey of Newgate has told the judge how many times the prisoner has been tried before, and therefore if he is found guilty, he certainly will be hanged. It is said to be the custom of the Old Bailey for one of the turnkeys of Newgate to give information to the judge how many times an old offender has been tried, by holding up as many fingers as the number of times the prisoner has been before arraigned at that bar.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
College * CollegiatesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes