Is "까마귀 고기를 먹었어?" some sort of idiom?
I came across this phrase and I understand the direct translation but it sounded so strange I figured it must be an idiom. They definitely weren't eating.. crows. Haha thank you!

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'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!

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

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

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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.

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