목불식정이 · 목불식정을
completely illiterate, unable to read even the simplest letter
Example
그 시절에는 목불식정인 사람이 적지 않았어요.
In those days, quite a few people couldn't read a single letter.
Word parts
목 eye+불 not+식 know+정 fourth stem
Unable to recognize even the simplest character, one with just two strokes.
Good to know
The native saying 낫 놓고 기역 자도 모른다 has the same meaning.