The scrotum. Also a nick name for a curate: a rude fellow meeting a curate, mistook him for the rector, and accosted him with the vulgar appellation of Bol—ks the rector, No, Sir, answered he; only Cods the curate, at your service.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cod Piece * Cod’s HeadNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes