I admit I was a baffled wondering about the origin of this expression until I read the explanation at a web site for “Historical Origins of English Words and Phrases.” The site explains that the “wash” was used in the 15^th century to describe “…waste liquid or food refuse from a kitchen…” that was used as food for domesticated animals and in particular as swill for pigs. The term evolved from a description for cheap food for pigs to describing cheap, poorly made liquor or “…poorly written manuscripts…” “In modern English, almost anything that is badly done or ridiculous can be equated to this term for barnyard slop. “