ETYM Old Eng. whennes, whens (with adverbial s, properly a genitive ending; -- see -wards), also whenne, whanene, AS. hwanan, hwanon, hwonan, hwanone; akin to Dutch when. Related to When, Hence, Thence.
From what place or origin or source; SYN. wherefrom.

From what place, source, or cause.