Use the word ninny for someone who's incredibly foolish — in other words, a dope or a nitwit. You might warn your brother not to be a ninny when he's just about to pop an entire hot chile in his mouth.
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.