A clerk in a counting house, whose sole business it is to clear or swear off merchandise at the custom-house; and who, it is said, guards against the crime of perjury, by taking a previous oath, never to swear truly on those occasions.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Damme Boy * DamperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes