Butt Monkey/Headscratchers


 * Why is it that "Butt Monkey" and "Ass Monkey" mean two completely different things, despite having almost identical names and "butt and "ass" being synonyms?
 * Etymology: "Butt Monkey" = a guy who's the butt of every joke. "Ass Monkey" = a guy who's an ass (coming from "Dumbass" rather than "Jerkass"), but in a funny way.