낫 놓고 기역자도 모른다

“Does not recognize the letter giyeok even with a sickle in hand.”

Someone is so uneducated that they miss even the most obvious clue right in front of them, since a sickle is shaped like the Korean letter giyeok (ㄱ).

— Traditional South Korea Wisdom, South Korea

Source: Traditional Korean proverb (속담), public-domain oral tradition