Whence, adv.
Etymology: [OE. whennes, whens (with adverbial s, properly a genitive ending; -- see -wards), also whenne, whanene, AS. hwanan, hwanon, hwonan, hwanone; akin to D. when. See When, and cf. Hence, Thence.]
1. From what place; hence, from what or which source, origin, antecedent, premise, or the like; how; -- used interrogatively. Whence hath this man this wisdom Matt. xiii. 54. Whence and what art thou Milton.
2. From what or which place, source, material, cause, etc.; the place, source, etc., from which; -- used relatively. Grateful to acknowledge whence his good Descends. Milton.
Note: All the words of this class, whence, where, whither, whereabouts, etc., are occasionally used as pronouns by a harsh construction. O, how unlike the place from whence they fell Milton.
Note: From whence, though a pleonasm, is fully authorized by the use of good writers. From whence come wars and fightings among you James iv. 1. Of whence, also a pleonasm, has become obsolete.
Shades of meaning
Fernald has no entry of its own for whence; he lists it under therefore.
Where he writes about it:
Wherefore is the correlative of therefore, and whence of hence or thence, appending the inference or conclusion to the previous statement without a break. — from therefore