From Wiktionary, the free dictionary
This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium . Old Korean 去 隱 等 隱 ( “ as for that which was... ” ) , but could not find primary source citations”
From Middle Korean -거든〮 Yale : -ketún ).
-거든 • (-geodeun )
( somewhat formal ) if ; provided (that)
Synonym: -으면 ( -eumyeon ) 시간 이 있 거든 같이 영화 보러 갑시다 .Sigan-i it-geodeun gachi yeonghwa boreo gapsida. If you have time, let's go to the movies together. ( literary ) given that ; especially used in rhetorical questions
Synonym: -는데 ( -neunde ) 벼 도 익으면 고개 를 숙이 거든 너 는 어찌 그리 오만방자하냐 ?byeo-do igeumyeon gogae-reul sugi-geodeun neo-neun eojji geuri omanbangja-hanya? Given that even rice plants bend their heads as they ripen, how can you be so conceited and arrogant?In the intimate style , a declarative ending explaining a reason.
내일 이 일요일 이 거든 요 .Naeir-i iryoir-i-geodeun -yo. It's because tomorrow is Sunday. 
