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.
His whereabouts are utterly unknown.
There is no information whatsoever about him.
Literary kanji form; 杳 is a rare character meaning 'dark' or 'distant'.
Irregular kanji usage; 沓 normally means 'shoes' or 'pile up', used here as ateji.
From the literary kanji 杳 (dark, distant) + quotative と + して, forming an adverbial phrase. The kanji 沓 is an ateji substitution.