have a hard head
1. To be inflexible or stubborn; to be overly willful or obstinate. Our son has such a hard head. Once he's made up his mind about something, there's no way anyone can change it!
2. To be very practical and shrewd; to not be persuaded or influenced by emotions. You have to have a hard head to work in this job—you can't let people's emotional appeals influence your decisions.
3. To be dim-witted, stupid, or an especially slow learner. I've always had a hard head when it comes to math.
4. Literally, to have an especially hard skull, often said humorously after receiving a mild blow to the head. A: "Oops! Sorry, I didn't mean to hit you!" B: "It's OK, I have a hard head!"