expression, adverb
nobody knows; unknown to anyone
Fixed expression used adverbially to mean that something is not known by anyone. Often appears in set phrases like 誰知らぬ顔 (a face nobody knows) or 誰知らぬ間に (before anyone knows).
彼は誰知らぬ顔で会議に出席した。
He attended the meeting with a face nobody knew.
誰知らぬ間に雨が降り出していた。
Before anyone knew it, it had started raining.