In Japanese, JK means Joshi Kousei 女子高生, "high school girl," or it can mean JouKou 常考, which translates to "obviously." In both cases, JK is a slang.
"High School Girl"
Most of the time, the meaning of JK will be "high school girl," joshi kousei 女子高生.
Manga: Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui! 私がモテないのはどう考えてもお前らが悪い! (Chapter 1)
- Context: Tomoko, a middle school girl, talks about what she's about to become.
- joshi kousei da!
JKだ!
[I'll] be a high school girl! - joshi kousei 女子高生 is gikun for JK in the panel above.
Pronunciation
When JK means "high school girl," it's pronounced jeikei ジェイケイ, which is just the katakanization of the English letters jei J and kei K. [JK - dic.nicovideo.jp, 2019-01-22]
But when it means JouKou 常考 it's pronounced JouKou ジョウコウ.
JouKou 常考
The word JouKou 常考, abbreviated JK, is an abbreviation of:
- joushiki-teki-ni kangaete
常識的に考えて
Think [using] common sense.
Normally. Naturally. Obviously.
It's common sense!
Everybody knows this!- joushiki 常識
Common sense. - joushiki-teki 常識的
Common-sense-cal. (na-adjective.) - joushiki-teki-ni 常識的に
Common-sense-cally. (adverb.) - kangaeru 考える
To think.
- joushiki 常識
Although this is a completely normal Japanese phrase, which you find naturally and randomly in the Japanese language, it's also a meme used by users of internet forums and image boards.[常識的に考えて - dic.nicovideo.jp, 2019-01-22]
In particular, JouKou 常考 isn't even a real word. It's solely an internet slang, an abbreviation of a meme phrase. Which makes JK an abbreviation of an abbreviation of a meme.
This stuff, joukou, etc. can be used honestly, to tell someone to think about it normally, to use common sense, but it can also used sarcastically, like by someone pretending their opinion is common sense, for example.
To have a better idea: Daru, from Steins;Gate, says JouKou a lot.
Manga: Danna ga Nani wo Itteiru ka Wakaranai Ken 旦那が何を言っているかわからない件 (Chapter 1, 合わない趣味とハマったソリ)
- Context: real life imitates art.
- karaage niwa mayoneezu daro
からあげにはマヨネーズだろ
To karaage, mayonnaise, right.- i.e. karaage goes well with mayonnaise.
- karaage - usually chicken deep-fried in oil.
- JK (joukou)
JK(じょうこう)
(it's common sense.) - mogumogu
もぐもぐ
*munch munch*
(onomatopoeia.) - JK (joukou) tte nanda...?
JK(じょうこう)ってなんだ・・・?
What's joukou...? - JK (joukou)...
JK(じょうこう)・・・
Joukou... - pi...
ピ・・・
*phone beep* - joshi kousei?
女子高生?
Highs school girl? - nande joshi koosee?
なんでジョシコーセー?
Why high school girl? - a, chigatta "joushiki-teki ni kangete..." ka
あ、違った「常識的に考えて・・・」か
Ah, [I got that wrong], "thinking with common sense..." huh. - iya karaage ni mayoneezu wa chigau daro
いやからあげにマヨネーズは違うだろ
[Wait,] no, [putting] mayonnaise on karaage isn't right, [what are you talking about].
Origin of JK
For someone who's not a Japanese native it can be easy to figure out how JK came to mean Joshi Kousei 女子高生: it's because of the romaji: Joshi (J) Kousei (K), JK.
However, note that, in Japanese, you don't use the Latin alphabet, you don't write in romaji, you use the Japanese alphabet. So for the average Japanese person the origin of the word would be a lot harder to understand.
To elaborate: there is no way to write J or K in Japanese. The minimum you can write is jo じょ, or even ji じ, and ko こ. With kanji, joshikousei is written as 女子高生. With hiragana, as じょしこうせい.
If you recall names of anime that have been abbreviated (Watamote, Oregairu, Oreimo, Konosuba, etc.), you'll notice that there is literally no Japanese abbreviation made only of consonants; they are always made of syllables.
So how come JK is a Japanese word? The answer is simple: internet. To talk on the internet you need a computer. And to type Japanese in a computer you normally use romaji. You type romaji which is converted to hiragana then kanji. So only computer people could have come up with JK.
Enjo Kousai 援助交際
Originally, JK was word used by people looking to date "school girls," joshi kousei 女子校生, in exchange for compensation ($$$), a practice labelled enjo kousai 援助交際, "support relationship," abbreviated en-kou 援交.
And yeah, that's kind of a crime.
Maybe that's why they chose such a secret code word, like JK, so that the average Japanese citizen wouldn't be able to understand: they wanted to avoid suspicion and the police.
Anime: Puni Puni☆Poemii ぷにぷに☆ぽえみぃ (Episode 2)
- A ganguro ガングロ gyaru ギャル, clinging to a salaryman sugar "daddy," papa パパ.
However, such people started using the word JK in public internet forums, where users who weren't after school girls browsed by. Obviously, they saw the word they didn't know what meant and asked: "what does that even mean?" Just like you'd ask if you read ROFLMAO for the first time on the internet.
Because of this, the JK criminal slang began being spread in internet forums as just another slang: it was used in place of joshikousei, "high school girl," but it wasn't necessarily used by people looking for compensated dating or prostitution. Its usage continued to spread across the internet, until it finally broke the ultimate barrier and the term JK began being used in the Real World™.
Suddenly, the code word used by old men looking for underage prostitutes was also being used by official organizations trying to look modern by using slangs and advertising stuff to JK's. Magazines were legitimately published with JK on their titles which, obviously, had nothing with criminal activity.
Since everybody uses the word JK as a slang now, it just means "high school girl," and has little to do with its original criminal usage.
Manga: Saiki Kusuo no Psi-Nan, 斉木楠雄のΨ難 (Chapter 183, Ψテク戦士100円マン!)
- Context: Saiki needs money, and decides that he will do ANYTHING for 100 yen using his super powers. His grandpa hears about this.
- tatoeba anna koto ya konna koto mo kanou jato iu koto ka~~~~!?
例えばあんな事やこんな事も可能じゃと言うのかーー!?
For example, [are you saying that] something like that and something like this is also possible~~?!- anna koto ya konna koto
あんなことやこんなこと
Something like that and something like this.
Vaguely refers to things to do without describing what they actually are.
This phrase is typically used in lewd jokes.
- anna koto ya konna koto
- kata-tataki
肩たたき
Shoulder massage. - kyacchibooru
キャッチボール
Catchball. - mimikaki ando hizamakura
耳かき&ひざまくら
Ear cleaning and lap pillow. - osanpo
お散歩
Walk. - soine
添い寝
Sleeping together. - paaaaaa
パァァァァァ
*awe* - boku ga JK (jeekee) dattara daimondai dazo
僕がJK(ジェーケー)だったら大問題だぞ
If I were a JK [that] would be a huge problem.
Some anime series feature a JK involved in enjo-kousai to spread awareness about the issue.
The same thing occurs with the issue of train "molesters," chikan 痴漢, who typically target female commuters like JKs gong to school and OLs going to work.
Manga: Africa no Salaryman, アフリカのサラリーマン (Chapter 2, アフリカの痴漢)
- Context: a kemono lioness and a gorilla JK are in a train with a toucan salaryman.
- chikan----!!!
痴漢ーーーー!!!
Molester!!! - kono tori no ossan oshiri sawatta!!
この鳥のオッサンおしり触った!!
This bird old-man touched [my] butt!! - e' nani?
えっ なに?
Eh, what? - saitee!! rorikon!!
サイテー!!ロリコン!!
[You're] the worst!! [You] pedophile!! (literally "lolicon," because they're underage high school girls.) - chigaimasu kedo jidankin-meate desu ka?
違いますけど示談金目当てですか?
[You got it wrong], is [your] objective settlement money?- chigau means "[it] differs," it's used to say someone is wrong when they claim something different from the reality.
- me-ate refers to what someone has set their eyes on, their aim, in this case the settlement money.
JC
The word JC means joshi chuugakusei 女子中学生, "middle school girl" in Japanese.
JS
The word JS means joshi shougakusei 女子小学生, "elementary school girl" in Japanese.
JD
The word JD means joshi daigakusei 女子大学生, "college girl" in Japanese.
Numbers: jk1, jc3, js2
Sometimes, a number if added after JS, JC, JK, JD. The number represents the school year. For example:
- JK1
joshi koukou ichinensei
女子高校1年生
High school first year girl. - JC3
joshi chuugaku sannensei
女子中学3年生
Middle school third year girl. - JS2
joshi shougaku ninensei
女子小学2年生
Elementary school second year girl.
In a way, this is similar to how abbreviated school years work in Japanese.
DK
After breaking through the cyber barrier, JK also ended up breaking through the gender barrier.
The word DK means danshikousei 男子校生, "high school boy" in Japanese.
It's such a rare word you can only figure out what it means when it's next to JK, and it's pretty much only used when JK is used too. (JK and DK = high school girls and boys). For most people, DK means Donkey Kong, not "high school boy."