The gallows, formerly consisting of three posts, over which were laid three transverse beams. This clumsy machine has lately given place to an elegant contrivance, called the NEW DROP, by which the use of that vulgar vehicle a cart, or mechanical instrument a ladder, is also avoided; the patients being left suspended by the dropping down of that part of the floor on which they stand. This invention was first made use of for a peer. See Drop.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Three-penny Upright * Three ThreadsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes