('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'.
(..) rig, there's a way inside. What is your name? / It's Nux. I'll pike her in the spine. Keep her breathing for ya!! No. Put a bullet in her skull. Stop the rig. Return my property... ...and I myself... ...will carry you to the Gates of Valhalla! (..)2015 Mad Max: Fury Road