Japanese body language -

頭が悪い[あたまがわるい, atama ga warui] be dumb; be slow

Yup, the Japanese can insult with the best of them.

When a person just doesn’t get it, can’t figure it out, can’t be taught, or is just plain slow to come around..s/he is said to have a bad (warui) head.

It’s different in the west. When a person is said to have a bad head, it means they are intent on doing wrong. They are evil. They can, in fact, be very sharp, just on the wrong side of the law.