noun
sworn younger brother; junior male peer
Refers to a male treated like a younger brother in a hierarchical relationship, such as in a gang, club, or workplace. Not a biological sibling.
彼は組長の弟分としてかわいがられている。
He is cherished as the boss's sworn younger brother.
サークルでは、彼が一番年下で
In the club, he is the youngest and everyone's junior peer.