'왜 이렇게 자꾸 까먹니?' 란 뜻입니다. 까먹다의 '까'와 까마귀의 '까'가 같은 것을 이용한 말 장난이에요. 까마귀 고기를 먹어서 뭘 자꾸 까먹느냐라고 살짝 비꼬는 것입니다. '까먹다'라는 말은 '기억에서 잊어버리다' 라는 뜻의 informal 한 표현입니다. 도움이 되었길 바래요!

I know what it means but I haven't ever use this expression for about 30 years in Korea.

Here is a fairy tale about the idiom http://terms.naver.com/entry.nhn?docId=4294808

Yes. It's kinda idiom. It means 'forget easily'. There was a long folk story. In a short.... long long long time ago, while a crow ran an errand from 염라대왕(King of Hell), he was busy to watch that people were hunting a horse. He lost to the important document in the show. Since then, people say 까마귀 고기를 먹었냐 to a person who forget easily.