절친 (jeolchin) — A very close friend.

Lesson 89 in our Korean Slang series, adapted from Korean Your Teacher Doesn’t Teach You by Injin Hwang. This expression appears in the book’s shortened expressions section.
What 절친 means
Use 절친 for a friend with whom you share a close bond. The shortened form comes from 절친한 친구. The situation and tone help determine how natural the expression sounds.
A short conversation
A: 두 사람은 언제 만났어?
B: 고등학교 때부터 절친이야.
A: When did you two meet?
B: We have been close since high school.
Use it with care
It need not mean your one and only best friend. Be careful not to label someone else’s relationship for them.
A neutral or polite alternative: 아주 친한 친구예요. — We are very close friends.
Try it yourself
Must you have known a 절친 since childhood?
Answer: No. The closeness matters more than the date. Read the Korean exchange aloud, then replace one detail with something from your own life.
Learn the rest of the series in Basic Korean Grammar. Find more expressions in the Korean slang book on Amazon.