middle brother
Refers to the brother who is neither the eldest nor the youngest among siblings. Used as a noun phrase.
彼は三人兄弟で、中の兄です。
He is the middle brother among three siblings.
Compound of 中 (naka, 'middle') + の (no, possessive particle) + 兄 (ani, 'older brother'). The exact historical derivation is uncertain, but the phrase follows a common pattern for describing sibling order.