Expresses a vague, unarticulated feeling or reason; often used adverbially with に (なんとなしに). Similar to なんとなく, but slightly more literary or introspective.
See also: なんとなく
I looked up at the sky for no particular reason.
His story felt somehow sad.
Alternate kanji spelling using 無; even less common than 何となし.
A fixed adverbial expression from 何と “as what; in what way” and 無し “there is none,” hence “with no particular reason or manner.”