jyaku niku kyoushoku
Today's Yo-ji is a good one to know for the world of fierce competition as it pertains to either sports or business. But an internet search reveals that its also commonly pops up in manga, drama, or movies about high school. Check the picture below and think Mean Girls.

Definition:
強い者が勝ち、弱い者が犠牲になること。
Translations:
1. Survival of the Fittest
2. The Law of the Jungle
3. A cut-throat struggle for domination
4. Dog-eat-dog
Plus your
Bonus word of the day:
生足
なま あし
nama ashi:
It means 'bare-legged' or 'bare-foot,' but is often used to refer to girls who are not wearing stockings,' and the fact that nama means raw carries a bit of a lecherous connotation. Use wisely.
Today's Yo-ji usage note: You can use it as a stand-alone concept, or attach の世界.
例文:JKナンパと言うことは弱肉強食だから、七転八起しないといけない。どうなっても、諦めなかったら、学校の一番もってもってやつになれるよ!
The world of JK Nanpa is dog-eat-dog. You have to keep trying, no matter how many times you fail. If you never ever give up, you can become the most desired boy in your school.*
*Feel free to use this as your own, Manga-Writers of Japan. You're welcome.