('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'.
(..) Don't talk to him like that. Here. You okay? Thank you. Um... How's your head? I mean, It's fine. Ah... I'm good. Ha. I've got a thick skull. I don't have a skull...Or bones. So... So Now what? Now what? Now what?! Oooh! What am I gonna do? (..)2013 Frozen