고인물 (goinmul) — A long-time expert or entrenched regular.

Lesson 57 in our Korean Slang series, adapted from Korean Your Teacher Doesn’t Teach You by Injin Hwang. This expression appears in the book’s fandom and entertainment section.
What 고인물 means
In a game or hobby, call someone a 고인물 when they know every shortcut and inside detail. The situation and tone help determine how natural the expression sounds.
A short conversation
A: 이 게임 숨은 길도 알아?
B: 응, 나 여기 고인물이야.
A: You know the hidden routes too?
B: Yes, I have played this forever.
Use it with care
Literally stagnant water. It can express admiration for skill or criticize a closed community; tone matters.
A neutral or polite alternative: 이 분야에 경험이 많아요. — I have lots of experience in this field.
Try it yourself
Is 고인물 always a compliment?
Answer: No. It may also criticize an unwelcoming, unchanging group. 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.