죽마고우
竹馬故友 · an old friend from bamboo-horse days
A friend since early childhood.
When to useIntroducing a friend you've known forever.
Examples
얘는 내 죽마고우야. 유치원 때부터 친구야.
This is my oldest friend — we've been friends since kindergarten.
Korean life dictionary
22 short guides to words and manners