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.
Can mean 'slow' in a broader sense (physically or mentally); 頭の悪い specifically targets intelligence.
Phrase composed of 頭 (head) + の (possessive particle) + 悪い (bad), literally 'bad-headed'. The expression is straightforward and modern.