妹いもうと分ぶん

noun

sworn younger sister; protegee treated as a younger sister

Refers to a younger female whom one mentors or protects and treats like a younger sister, often in hierarchical or traditional settings (e.g., geisha houses, yakuza, or close-knit groups).

• 彼かの女じょは私わたしの妹いもうと分ぶんで、何なにかと面めん倒どうを見みている。

  She is my sworn younger sister, and I look after her in various ways.

• 先せん輩ぱい