The bill of a bird, and the slit of a pen. Figuratively, the face and mouth of a woman; as, She holds up her neb: she holds up her mouth to be kissed.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Nazy * Neck StamperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes