expression
Fixed expression emphasizing that something is exactly that and nothing else, often used before a noun or clause. Similar to 他ならない but slightly more literary.
See also: 他ならない
彼の成功は他ならぬ努力の結果だ。
His success is nothing but the result of hard work.
この問題を解決できる
The one who can solve this problem is none other than you.
他ならない is the more common dictionary form; 他ならぬ is a slightly more literary or emphatic variant with the same meaning.
From 他 (ほか, 'other') + ならぬ (archaic negative of なる, 'to be'), literally 'it is not other'. The phrase fossilized as a set expression meaning 'none other than'.