expression, adverb
utterly unknown; completely without information
Literary expression used when someone's whereabouts or fate is completely unknown; often in the pattern 杳として知れない or 杳として消息がない.
彼の行方は杳として知れない。
His whereabouts are utterly unknown.
消息は杳としてない。
There is no information whatsoever about him.
From the literary kanji 杳 (dark, distant) + quotative と + して, forming an adverbial phrase. The kanji 沓 is an ateji substitution.