막역지우가 · 막역지우를
a bosom friend, a very close friend
Example
그와 나는 어릴 때부터 막역지우로 지내 왔어요.
He and I have been bosom friends since childhood.
Word parts
막 none+역 oppose+지 of+우 friend
Literally 'a friend with no friction between you'.
Good to know
Pronounced [마격찌우]; 막역한 사이 'close relationship' is common too.