The Collaborative International Dictionary of English v.0.48:

Whence \Whence\, adv. [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.]
   [1913 Webster]
   1. From what place; hence, from what or which source, origin,
      antecedent, premise, or the like; how; -- used
      interrogatively.
      [1913 Webster]

            Whence hath this man this wisdom?     --Matt. xiii.
                                                  54.
      [1913 Webster]

            Whence and what art thou?             --Milton.
      [1913 Webster]

   2. From what or which place, source, material, cause, etc.;
      the place, source, etc., from which; -- used relatively.
      [1913 Webster]

            Grateful to acknowledge whence his good
            Descends.                             --Milton.
      [1913 Webster]

   Note: All the words of this class, whence, where, whither,
         whereabouts, etc., are occasionally used as pronouns by
         a harsh construction.
         [1913 Webster]

               O, how unlike the place from whence they fell?
                                                  --Milton.
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   Note: From whence, though a pleonasm, is fully authorized by
         the use of good writers.
         [1913 Webster]

               From whence come wars and fightings among you?
                                                  --James iv. 1.
         [1913 Webster] Of whence, also a pleonasm, has become
         obsolete.
         [1913 Webster]

WordNet (r) 3.0 (2006):

whence
    adv 1: from what place, source, or cause

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "whence":
   accordingly, away, because of that, because of this, consequently,
   derivation, ergo, for that, for that cause, for that reason,
   for this cause, for this reason, for which reason, forth, fountain,
   hence, hereat, inception, off, on that account, on that ground,
   on this account, origin, out, propter hoc, provenance, provenience,
   root, then, thence, thereat, therefor, therefore, therefrom,
   thereof, thus, thusly, thuswise, well, wellspring, wherefore,
   wherefrom