be not much between the ears

To be rather stupid, dull, or dimwitted; to not have much common sense or basic intelligence. She seems nice enough, but I get the sense that there might not be much between the ears. Jake's boyfriend is incredibly attractive, but there isn't much between the ears from what I can tell. A: "See that guy? He's been pulling on a door marked 'Push' for the last five minutes." B: "Wow, there's not much between the ears there, huh?"