between your ears

between the/(one's) ears

In the head where the brain is (or should be). Often used in the phrase "have nothing between the/(one's) ears" to suggest that one does not have much common sense or basic intelligence. She seems nice enough, but I get the sense that she has nothing between her ears. Jake always goes out with really attractive guys that have nothing between the ears.
See also: between, ear
Farlex Dictionary of Idioms.

between your ears

or

between the ears

INFORMAL
If someone has nothing or little between their ears or between the ears, they are not clever. Footballers often have a reputation of not having much between the ears. He doesn't have a lot between the ears. Note: These expressions are usually used light-heartedly.
See also: between, ear
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • between the/(one's) ears
  • in donkeys
  • get (one's) ears lowered
  • prick up
  • pop (one's) ears
  • prick up (one's) ears
  • prick up ears
  • prick up one's ears
  • prick up one's ears, to
  • prick up your ears