Ninny is apt for someone who's silly and stupid, but it's also insulting and should be used with care. There's a certain gentle, teasing quality to ninny (as opposed to a word like idiot), but you're still going to get in trouble if you call your math teacher a ninny for giving the class a pop quiz. The word's origin is a bit of a mystery, though it may come from an innocent or the Italian word for "child," ninno.