Meaning:

“制服” means “uniforms,” and JK is short for the Japanese words “joshi koukousei” (girl students in high schools). Therefore, this term coined by Chinese netizens refers to schoolgirl outfits inspired by Japanese high school uniforms. A set of “JK uniforms” normally includes a cute pleated skirt, normally high waist and often in plaid patterns, a short-sleeved shirt with a bowtie matching the skirt for summer. The uniforms will include a long-sleeved shirt, waistcoat and coat for other seasons. JK uniforms are commonly seen in Japanese comics and have become increasingly popular in China as a cute casual daily outfit.

Example:

A: 你昨天发的朋友圈，照片中的小姐姐是谁呀？

Nǐ zuótiān fā de péngyǒuquān，zhàopiān zhōng de xiǎojiějiě shì shuí ya？

Who’s the girl in that photo you shared on WeChat Moments yesterday?

B: 你说的是穿JK制服的女生吗？她是我的小学同学。

Nǐ shuō de shì chuān JK zhìfú de nǚshēng ma？Tā shì wǒ de xiǎoxué tóngxué。

Are you talking about the girl in JK uniforms? She’s my primary school classmate.

A: 她长得真美。

Tā zhǎng de zhēn měi。

She’s very pretty.