Continuing dispute between the West Cumbrian towns of Whitehaven and Workinton inhabitants. The northern-most town calls the other this because the miners used to eat it in the pits or something.
The real reason is that at one time the town of Workington was richer than Whitehaven and they used to be able to afford the luzury of Jam. Hence the phrase was coined in Whitehaven who were jealous of their Workington pals.
Y' hoofing jam eater, git back to where thou cum fray!