nikushokukei 肉食系

In Japanese, nikusokukei 肉食系 means a kind of person that assertively, aggressively or even predatorily seeks romantic or sexual relationships.

It comes from nikushoku 肉食, literally "meat-eating," which is the term for "carnivore" animals, predators that go after prey. Plus the -kei ～系 suffix, "class," so nikushoku-kei, "carnivore-class."

Normally, society assumes guys chase girls. Guys chase skirt. And girls are the ones being chased. They're the pure "maidens," otome 乙女. And, in fact, some want to be chased. They don't want to be proactive, take the first step, and confess to the guy they like. They want the guy they like to confess to them, so they can feel their worth recognized.

So guys are assumed to be nikushokukei by default. In which case, it becomes redundant to call a guy nikushokukei. When nikushokukei is used, it tends to mean women that take the first step instead of men. Women that do go after the guys they want. Women that make the cheesy pick-up lines and so on. This is despite nikushokukei being technically gender-neutral.

Conversely, the term soushokukei 草食系, from "herbivore," is used when you have a guy that's too shy to ask girls out and must be asked out instead by a nikushokukei girl or something like that.

If we were to be explicit to avoid any ambiguity:

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

Note that in Japanese the word bitch ビッチ, meaning "slut," refers to a woman who would have sex with anyone. This isn't the same thing as nikushokukei for two reasons:

1. Just because a woman is assertive romantically that doesn't mean she's promiscuous. She might simply be assertively dating her one and only love. Who might be a soushokukei that would never muster the courage to confess to her, so she has to take the matter in her own hands or the ship won't sail.
2. A sexually promiscuous woman doesn't necessarily go after guys, it could be simply that she accepts any guy that goes after her. So soushokukei and bitch aren't mutually exclusive.

In case you're wondering: yes, these words are used toward real people. They're not technical terms, they're kind of like slangs, but they are used toward real people.

Within the anime fandom, one thing that's not exactly the same thing, but helps illustrate it, is the term oneshota おねショタ. It refers to a pairing (ship) of one "older girl," oneesan お姉さん, with a "young boy (character)," shota ショタ. Maybe because the girl is older, she ends up taking the lead in the ship, so it's like the girl is nikushokukei.

Also in the anime fandom, seme and uke can refer to who's assertive and who's passive in a heterosexual ship, although they're more commonly used with homosexual ships. 

By the way

• niku 肉
  Meat.
• taberu 食べる
  To eat.
• nikushoku doubutsu 肉食動物
  Meat-eating animal. Carnivore animal.
• gyaku-nanpa 逆ナンパ
  Girls hitting on guys. Reverse nanpa.
• nanpa ナンパ
  Hitting on someone. Generally guys hitting on girls.

Also note that kuu 食う is a more colloquial word for "to eat" that's also a sexual innuendo:

• onna wo kuu 女を食う
  To eat a woman. (literally.)
  To have sex with a woman. (innuendo.)