A mode of stealing in houses that let lodgings, by rogues pretending to be postmen, who send up sham letters to the lodgers, and, whilst waiting in the entry for the postage, go into the first room they see open, and rob it.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Dingey Christian * DipNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes