not very smart; slow-witted — a colloquial expression meaning someone is poor at understanding or reasoning (can be insulting).
彼は冗談で『自分は頭が悪い』と言ったが、実際はよく考えている。
He joked 'I'm dumb,' but in fact he thinks things through.
勉強しないままだと、頭が悪いと言われても仕方がない。
If you don't study, it's no wonder people call you dumb.
不用意に『頭が悪い』と人に言うのは失礼だ。
It's rude to casually tell someone they're dumb.