This is the 2476th most frequent Korean word.
덜
"덜" means "less" or "not completely" in English.
The word '덜' indicates 'less', here specifying the apple is 'less ripe' or 'unripe'.
나는 덜 익은 사과를 먹었다.
I ate an unripe apple.
The word '덜' is used to convey 'less' as in 'less important', modifying 'important'.
덜 중요한 과제를 나중에 끝냈다.
I finished the less important assignment later.
Here, '덜' serves to mean 'less', qualifying 'complicated'.