A portmanteau of "Canadian" and "fuck." Coined in the early days of Canada's founding by people displeased with the formation of Canada as a nation in the late nineteenth century. Primarily a slur. Still used in Northern USA states as a slur, but has become a somewhat affectionate term in modern days.
"Damn canuck just u-turned in the middle of the road!"
"Oh I dunno, those canucks ain't so bad, I mean have you ever had poutine?"