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. A: "Did you hear that Sam got promoted to General Manager?" B: "You've got to be kidding me! But she has such a thick skull—how is she going to lead a whole team?" Of course my partner for the assignment has a real thick skull—meaning I'm going to have to do everything myself!

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! Good luck even making a suggestion to Tiffany—she has quite a thick skull! You really have a thick skull, you know that? It wouldn't kill you to let other people make decisions from time to time!

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!" Luckily, Tommy has a thick skull, so the doctor says he'll be just fine once that lump goes down. A: "Don't worry, I have a thick skull, I'll be fine." B: "Even so, that was quite a blow. As your coach, I have to have one of the trainers examine you."