also: なのめならず
expression
unusually; extraordinarily; remarkably
A somewhat literary or formal fixed expression meaning 'not just a little' or 'to an unusual degree'. Often used with verbs like 驚く, 感心する, or adjectives describing a notable quality.
彼の演奏は斜めならず素晴らしかった。
His performance was extraordinarily wonderful.
その知らせを聞いて、斜めならず驚いた。
Hearing the news, I was unusually surprised.