Colloquial expression meaning slow-witted or not very smart; used to say someone is poor at understanding or reasoning.
そんな簡単な問題も分からないなんて、頭悪いんじゃない？
You can't even solve such a simple problem — aren't you kind of slow?
彼はいつも注意力散漫で、たまに本当に頭悪いと思う。
He's always absent-minded; sometimes I really think he's not very bright.
テストでミスが多くて、自分は頭悪いと落ち込んだ。
I made many mistakes on the test and felt down, thinking I was dumb.