Northumberland, from the particular croaking in the pronunciation of the people of that county, especially about Newcastle and Morpeth, where they are said to be born with a burr in their throats, which prevents their pronouncing the letter r.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Croaker * CrockersNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes