Goinmul is Korean Internet slang derived from the word for "still water". The term is used to refer to games that are inactive, stagnant, or in decline, as well as players of those games. Generally, these are games with a low influx of new players due to a perceived high barrier of entry, whether due to the game's age or its reputation for being difficult.
While this word used to have a negative connotation, starting in the 2010s, goinmul has been used respectfully to describe high-level players that have stuck by games despite their perceived barrier of entry and amassed tremendous knowledge/skill/experience in them.
Examples of Goinmul games include:
Competitive games prior to the ubiquity of online updates used to ship as is, and players had to live with the state of the game's balance until the next game in the series came along, sometimes more than a year later. Players would try things out until they found something that was considered to be strong, and other players would try to find techniques to counteract that playstyle. Nothing about the game had changed between launch and the end of the competitive lifespan. The only thing that had changed was the players' understanding of the game, and thus the metagame evolved organically.
This is no longer the case. Most modern-day competitive games see constant patches from the developers over their competitive lifespan. Things that are too strong are nerfed, and things that are too weak are buffed. Characters get new moves or items every season, and new DLC characters are introduced regularly. The game is evolving, and because of that, player understanding is constantly being reset by developer changes, big and small.
Players who desire stability in the competitive games they play may prefer to stick with goinmul games, as they are past the window of developer support and unlikely to see balance patches that destabilize existing players.