A person that guts letters; i.e. opens them and secretes the money. The kiddey was topped for the poultry rig; the young fellow was hanged for secreting a letter and taking out the contents.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
poulain * poundNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes