noun, used as a suffix
Familiar suffix attached to a name or title, used mainly by younger siblings or close acquaintances to address or refer to an older brother figure. Often seen in forms like お兄さん or 兄ちゃん.
See also: お兄さん
太郎兄、ちょっと手伝ってくれる？
Taro-nii, can you help me a bit?
彼は僕のことを「弟」と呼ぶけど、僕は「兄」と呼んでいる。
He calls me 'little brother,' but I call him 'older brother.'
Derived from the older brother term 兄 (あに), with a sound shift to にい for suffix use. The exact historical derivation is uncertain.