The Collaborative International Dictionary of English v.0.48:

Vanquish \Van"quish\ (v[a^][ng]"kw[i^]sh), v. t. [imp. & p. p.
   Vanquished (v[a^][ng]"kw[i^]sht); p. pr. & vb. n.
   Vanquishing.] [OE. venquishen, venquissen, venkisen, F.
   vaincre, pret. vainquis, OF. veintre, pret. venqui, venquis
   (cf. an OF. infin. vainquir), fr. L. vincere; akin to AS.
   w[imac]g war, battle, w[imac]gend a warrior, w[imac]gan to
   contend, fight, OHG. w[imac]gant a warrior, w[imac]gan to
   fight, Icel. v[imac]g battle, Goth. weihan to fight, contend.
   Cf. Convince, Evict, Invincible, Victor.]
   1. To conquer, overcome, or subdue in battle, as an enemy.
      --Hakluyt.
      [1913 Webster]

            They . . . vanquished the rebels in all encounters.
                                                  --Clarendon.
      [1913 Webster]

   2. Hence, to defeat in any contest; to get the better of; to
      put down; to refute.
      [1913 Webster]

            This bold assertion has been fully vanquished in a
            late reply to the Bishop of Meaux's treatise.
                                                  --Atterbury.
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            For e'en though vanquished, he could argue still.
                                                  --Goldsmith.
      [1913 Webster]

   Syn: To conquer; surmount; overcome; confute; silence. See
        Conquer.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vanquish \Van"quish\, n. (Far.)
   A disease in sheep, in which they pine away. [Written also
   vinquish.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

vanquish
    v 1: come out better in a competition, race, or conflict;
         "Agassi beat Becker in the tennis championship"; "We beat
         the competition"; "Harvard defeated Yale in the last
         football game" [syn: beat, beat out, crush, shell,
         trounce, vanquish]