loony binner
Someone who belongs in a loony bin but with the added subtle implication that even when a person appears fully functional and sane in the world that the truth can be entirely otherwise.
As I sat at my computer workstation on the other side of the wall from where the shrieking mentally ill prison inmate was coming entirely unglued, performing all of my workplace tasks without difficulty, a novel question popped suddenly into my head: just who exactly is the loony binner?