have a thick skull
1. To be dim-witted, stupid, or an especially slow learner. I've always had a thick skull when it comes to math.
2. To be inflexible or stubborn; to be overly willful or obstinate. Our son has such a thick skull. Once he's made up his mind about something, there's no way anyone can change it!
3. Literally, to have an especially dense skull, often said humorously after receiving a mild blow to the skull. A: "Oops! Sorry, I didn't mean to hit you!" B: "It's OK, I have a thick skull!"