1. What does it mean?
썩은물, literally rotten water, pushes the metaphor of 고인물 even further. It can jokingly describe a veteran whose detailed knowledge or practiced technique makes everyone else look new.
Also written or said as sseogeunmul
2. The nuance
The exaggeration can be admiration or criticism of an insular old guard. It is not a neutral job title, and calling a stranger rotten may sound hostile without the shared joke.
3. Examples
“발소리만 듣고 지도를 맞히다니 완전 썩은물이네.”
Recognizing the map just from the footsteps? That's extreme veteran-level knowledge.
“나도 오래 했지만 그 숨겨진 길은 몰랐어. 썩은물의 세계다.”
I've played a long time too, but I didn't know that hidden route. That's a whole other level of experience.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.