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.
between your ears
or between the ears
INFORMALIf 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.