낫 놓고 기역 자도 모른다
sickle lay-AND ‘giyeok’ letter doesn’t-know
[…like] laying down a sickle and not recognizing the letter ‘giyeok.’
[giyeok is written ㄱ]
This means “he doesn’t know his ass from a hole in the ground” and variations on that theme. It’s based on the idea that the Korean letter named ‘giyeok,’ which is written ㄱ, is evidently a sickle shape. To not know that is to be illiterate, and thus not particularly well-informed.