one's younger brother
Humble or formal term for one's own younger brother, often used in letters or formal speech. Contrasts with 舎兄 (one's elder brother).
See also: 舎兄
My younger brother will graduate from university next year.
sworn younger brother; underling; junior male peer
In yakuza or similar hierarchical groups, refers to a junior member who has sworn loyalty. Also used more broadly for a junior male peer in certain traditional or organizational contexts.
As the boss's sworn younger brother, he plays an important role in the organization.
front company for organized crime
From Middle Chinese-derived kanji 舎 (house, dwelling) and 弟 (younger brother), originally a humble way to refer to one's own younger brother within the family. The yakuza usage developed as an extension of the familial metaphor.