간담상조가 · 간담상조를
close, heart-to-heart friendship
Example
두 사람은 간담상조하는 사이라 비밀이 없어요.
The two are such close friends that they keep no secrets.
Word parts
간 liver+담 gallbladder+상 mutual+조 shine
Literally 'liver and gall shown to each other': revealing your innermost self.