NAB,
a Hat, Cap, or Head, also a Coxcomb. Ill nab ye, I'll have your Hat or Cap. Nim the Nab, steal the Hat or Cap. Nabbed, apprehended, taken or arrested.
Nab in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).