('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'.
(..) Worked the night shift. Good place for dinner when we're through here, Sam. No one came to claim the body. No known next of kin. Crushed skull. Probable cause, repeated blows to the back of the head... by a blunt, obtuse-angled object. (..)1992 Twin Peaks: Fire Walk with Me