A colloquial form of 格好の悪い; means uncool, unstylish, or embarrassing (informal).
そのシャツ、似合わないよ。ちょっとかっこ悪い。
That shirt doesn't suit you. It's kind of uncool.
昔の写真を見て、かっこ悪いと思った。
I looked at an old photo and thought it looked uncool.
そんなふるまいをすると本当にかっこ悪いよ。
Acting like that really makes you look bad.