In Korean 흑기사 ("Black Knight"), this word would mean someone (generally a guy) who volunteers to take a drink for another person (generally a girl). For a man it’s 흑기사 (heuk-gi-sa, “dark knight”) or 흑장미 (heuk-jang-mi, “black rose”) for women. One can ask someone to be their black knight/rose, or someone can offer to do it, but once you’ve done it, you’re theirs for the rest of the night. If you ask someone and they refuse, then you have to take two drinks.
While playing a drinking game, a girl has had too much to drink but has to take a shot. She can then call out for a Black Knight or Dark Knight and a generous guy may take the drink for her.