The mouth. Gift of the gab; a facility of speech, nimble tongued eloquence. To blow the gab; to confess, or peach.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Gab StringNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes