From Lobscouse, a sailors meal of boiled meat, vegatables and hardtack common to the sailing trade of Liverpool. My description may make the actual meal sound more appealing than it frequently was.
The Liverpool economy was built on shipping for a couple of hundred years. They were a major scource of vessels and crews for the slave trade before Bitains decided that slavery was a bad thing.
It's a simple jump to transfer the name of meal to a mild jibe about the folks living there.
The same process as Jack Tars becoming Limeys...or is that too recursive?