Another way of saying "Middle Of Nowhere", generally implying a negative tone, and the Idaho reference because the state is seen as boring. Often used if some one lives in a boring area or they got lost going somewhere.
Guy 1: Dude, where are you? You're late
Guy 2: I took and wrong turn and ended up in Buttfuck Idaho
Girl 1: So you got anyplans for the weekend?
Girl 2: No, I live in Buttfuck Idaho