개천에서 용 난다
A dragon rises from a small stream.
Used to describe a great man with humble beginnings.
Oriental myths talk about dragons rising up from the depths of the sea, so it would be unexpected and quite amazing if one were to rise up from a shallow stream.
미천한 집안에서 뛰어난 인물이 나는 경우에 이르는 말
From rags to riches.