('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'.
(..) Tier south, clear! Man missing on tier , cell ! Dufresne! Come out. You're holding up the show! Don't make me come down or I'll thump your skull for you! Damn it, you're putting me behind! I got a schedule to keep. You'd better be sick or dead in there. I shit you not! You hear me? (..)1994 The Shawshank Redemption
(..) So? So I'm a rock hound. At least I was in my old life. I'd like to be again. Or maybe you'd like to sink your toy into somebody's skull. No, I have no enemies here. No? Wait a while. Word gets around. The Sisters have taken quite a liking to you. (..)1994 The Shawshank Redemption