'Orange. Tell me, boy, does anything penetrate that thick skull of yours? Didn't you hear me say, quite clearly, that only one rat spleen was needed? ...'
As I know, "thick skull" means "slow to learn; stupid". But what does "does anything penetrate that thick skull of yours" imply? Does it mean he had learnt something wrong or something got into one's thick skull and made him crazy/abnormal or something? What does it mean exactly?
-- From Harry Potter.