국룰 (gukrul) means the thing everybody does, the default nobody voted on. 치킨엔 콜라가 국룰 means chicken comes with cola, and that is simply how it is.
The word is 국민 (gungmin, the citizenry) plus 룰, the English “rule.” A national rule, which is a joke, because nothing it describes was ever a rule.
Pronunciation is worth flagging. Written 국룰 and pronounced 궁뉼, roughly gung-nyul. The ㄱ before ㄹ turns into an ng sound and the ㄹ turns into an n. If you search gukrul you will find it, and if you listen for gukrul you will never hear it.
Usage
치킨엔 콜라가 국룰이지
Chicken goes with cola. The classic example.
이건 국룰이야
This is just how it’s done, applied to whatever is being argued about.
국룰 아님? (gukrul anim?)
Isn’t this standard? Used to check whether your assumption is shared, and the -음 ending marks it as internet writing.
영화 볼 땐 팝콘이 국룰
Popcorn with a film.
The claim it is making
국룰 asserts consensus. That is the interesting part. Nobody says 국룰 about something uncontroversial, because there would be no reason to. It gets said when the speaker suspects the other person might disagree and wants to place the burden on them.
Which means it is a rhetorical move as much as a description. 이건 국룰이야 is a way of ending a discussion by claiming everyone is already on your side.
That also makes it playful rather than serious. Nobody deploys 국룰 for anything that matters. The subject is almost always food pairings, small social habits, or the correct order to do something trivial in.
국룰 vs 정석 vs 기본
Three words that all mean something like “the standard way,” and they are not interchangeable.
정석 (jeongseok) means the textbook method, the technically correct approach. It comes from the game of go, where it refers to established opening sequences. 정석대로 하다 is to do it by the book. It implies there is a right answer and expertise behind it.
기본 (gibon) means basic, the minimum, the foundation. 기본은 해야지 means you should at least do the basic version.
국룰 has no authority behind it at all. It is popular consensus and nothing else, and the humor comes from dressing that up in the language of national law.
So a cooking method has a 정석. Chicken and cola is 국룰. The difference is whether anyone could point to a source.
Where it goes wrong
Do not use it to settle an actual disagreement. Calling your position 국룰 in a real argument is a way of refusing to give a reason, and the word is light enough that doing so reads as dismissive rather than as a point.
Do not use it about cultural practices to a Korean audience if you are learning the language. This one is specific: a learner saying 한국에선 이게 국룰이죠 sounds like generalizing about a country from limited exposure, even when the observation is correct. Koreans have wide latitude to make sweeping claims about Korea and visitors have less, which is not fair but is how it lands.
Also watch for the near-homophone in writing. 국물 (gukmul) means broth and is a completely different word. In fast typing they get confused, and food posts are the exact place where both would plausibly appear.
Formal settings
Not in writing. The word is a joke construction, and using it in a document would be like writing “it’s basically the law” in a memo.
It has spread far enough that it appears in advertising copy and TV subtitles, so it is not obscure. It is still firmly casual, and the age skew is real: it arrived around 2019 and sounds natural from people who were online then.
The formal alternatives are 관례 (gwallye, customary practice) or 일반적으로 (ilbanjeogeuro, generally). Both are dry, which is the correct register when you are writing something down.
For actual use, keep 국룰 for food and small habits, and let a Korean speaker be the one to declare what the national rule is.