From Longman Dictionary of Contemporary Englishthick-headedˌthick-ˈheaded adjective informal STUPID/NOT INTELLIGENTextremely stupid He’s so thick-headed he can’t understand simple instructions.
Examples from the Corpus
thick-headed• In the morning I awoke, thick-headed and a prisoner.• Who could have seen through such thick-headed nonsense?• I got up, thick-headed, with a cloying mouth.• He was stiff with mud and thick-headed with tiredness as he and Isay trooped back through the gatehouse to the Manse. 