soushokukei 草食系

In Japanese, soushokukei 草食系 means a kind of person that's passive in romantic or sexual relationships. Generally, it means a guy that's too shy or weak-willed to ask out and date a girl he's interested in.

It comes from soushoku 草食, literally "grass-eating," which is the term for "herbivore" animals, the prey that the predators go after. Plus the -kei ～系 suffix, "class," so soushoku-kei, "herbivore-class."

Although soushokukei is gender-neutral and can qualify a woman too, traditionally, it's women that are asked out by guys, so they're assumed to be soushokukei by default. It'd be redundant to call a woman soushokukei, so it tends to be a guy who's soushokukei instead.

Conversely, the term nikushokukei 肉食系, from "carnivore," refers to someone who's romantically assertive, that is, what guys are assumed to be, so it tends to refer to romantically assertive women.
If we were to be explicit to avoid any ambiguity:

• soushoku-kei danshi 草食系男子
  soushoku danshi 草食男子
  Herbivore-class guy.
  Herbivore guy.
• soushoku-kei joshi 草食系女子
  soushoku joshi 草食女子
  Herbivore-class girl.
  Herbivoregirl.
• nikushoku-kei danshi 肉食系男子
  nikushoku danshi 肉食男子
  Carnivore-class guy.
  Carnivore guy.
• nikushoku-kei joshi 肉食系女子
  nikushoku joshi 肉食女子
  Carnivore-class girl.
  Carnivore girl.

Besides these, there are also:

• zesshoku-kei danshi 絶食系男子
  zesshoku danshi 絶食男子
  Fasting-class guy.
  A guy with no romantic interest. Doesn't "eat."
• zasshoku-kei danshi 雑食系男子
  zasshoku danshi 雑食男子
  Omnivorous-class guy.
  A guy who's both soushoku and nikushoku somehow.
  Basically, if girls think nikushoku is too assertive and soushoku is too passive, zasshoku is the perfect balance between the two.

By the way

• kusa 草
  Grass.
  • Also means "lol," see: kusa 草 internet slang.
• taberu 食べる
  To eat.
• soushoku doubutsu 草食動物
  Grass-eating animal. Herbivore animal.

In recent years, with Japan's declining birth rate, and now declining population, soushokukei became a popular term to blame a generation of guys that's not being assertive enough to make those babies and keep the next generation's workforce staffed.

Now, whether this is because of economic anxiety or just because there are too many guys who became otaku オタク and then decided a 2D waifu is as good or better than a 3D girl in flesh, I don't know. That's not the theme of this site.

All I know is that soushokukei is one of the many Japanese words referring to personalities of people. Because Japanese people love making up words to refer to personalities.

Just like tsundere, or, something that's more relevant to this article: literally just the letter M, short of "masochist" personality, which's used in ways in Japanese you wouldn't use the word "masochist" in English. See do-M ドＭ reference.