1. What does it mean?
Literally evoking water that has stayed in one place, 고인물 labels someone who has spent a long time in a game or community. It is often used in admiration of seemingly effortless skill.
Also written or said as 고인 물, goinmul
2. The nuance
The same image can criticize a stagnant or unwelcoming group. A playful compliment about experience and a complaint about gatekeeping are different uses; do not assume the word is always praise or always an insult.
3. Examples
“지도 안 보고 숨겨진 길까지 찾는 걸 보니 고인물이네.”
You found even the hidden path without checking the map. You're clearly a veteran.
“고인물끼리만 아는 농담 말고 새 멤버도 이해하게 설명해 줘.”
Explain it so the new members understand, instead of using jokes only the old regulars know.
Sources
Korea Creative Content Agency: gaming vocabulary
Nate News: Korean usage in the press
References support the meaning and usage. Example sentences are written for NoCapWiki.