A jeering appellation for a north country seaman, particularly a collier; Jock being a common name, and crowdy the chief food, of the lower order of the people in Northumberland.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Jobbernole * To JockNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes