('informal') You need to 'get it into/through your thick skull' [=you need to realize/understand] that you can't just sit there and expect everyone else to do your work for you!
('informal') He can't seem to 'get it through his skull' that I don't like him.
He had a fractured 'skull'.
(..) - Amassed on the cliffs. Their numbers are swollen. They say that the king's standard flies again. Then let them come, let them break their skulls on these stone walls. And bring me my prize. My Lord, we only have a few hours before the tide comes on. Not long enough to breach the castle walls. (..)2012 Snow White and the Huntsman