i-adjective
slow; dim-witted; not bright
Colloquial and somewhat blunt way to say someone is not intelligent or slow on the uptake. Often used in casual, critical remarks.
See also: 頭が悪い
あの人は頭の悪いから、説明しても無駄だよ。
That person is dim-witted, so it's useless to explain.
頭の悪い質問で、先生を困らせてしまった。
I troubled the teacher with a stupid question.