This is the 6195th most frequent Korean word.
숨긴
"Hidden."
Here, '숨긴' is used as the past participle form of 'to hide' (숨기다) to describe the act of concealing documents in the past.
그는 중요한 서류를 서랍에 숨긴 것이 들켰다.
He was caught hiding important documents in the drawer.
In this sentence, '숨긴' describes truths that were hidden, indicating that the act of hiding was completed before the moment of speaking.
나는 친구에게 숨긴 사실을 모두 털어놓았다.
I confessed all the truths that I had hidden from my friend.
The word '숨긴' here is used as an adjective to describe the painting that was hidden.