Yooper is a bastardization of the term U.P.er.
And is "U.P.er." a transmorphigation of 'Up Here !' ? As in 'I'm writing from valhalla' or something .... and don't tell me that's weird because you have viking ascendance Mr Olaf Rosenberg !!!
Upper Peninsula, as in the upper half of the state of Michigan (although I think some Wisconsiners (ites?) use it if they live in the northern part.
I always knew Cthulu was up there.