Using "Korean" in the phrase as a reference to Korean's cliché attribute of hardcore gaming, and their many releases of mmorpgs that implicate such gameplay.
Examples of Korean grind fest:
Sword Of The New World: Granado Espada
Boy-2: I've played it before, even though quests in this game would give you more experience than repeatedly killing the same foes over and over again, the quest still requires you to kill the same foe over and over again.
Boy-1: Oh, so it's just another Korean Grind Fest.