The Collaborative International Dictionary of English v.0.48:

defy \de*fy"\ (d[-e]*f[imac]"), v. t. [imp. & p. p. Defied
   (d[-e]*f[imac]d"); p. pr. & vb. n. Defying.] [F. d['e]fier,
   OF. deffier, desfier, LL. disfidare to disown faith or
   fidelity, to dissolve the bond of allegiance, as between the
   vassal and his lord; hence, to challenge, defy; fr. L. dis- +
   fides faith. See Faith, and cf. Diffident, Affiance.]
   1. To renounce or dissolve all bonds of affiance, faith, or
      obligation with; to reject, refuse, or renounce. [Obs.]
      [1913 Webster]

            I defy the surety and the bond.       --Chaucer.
      [1913 Webster]

            For thee I have defied my constant mistress. --Beau.
                                                  & Fl.
      [1913 Webster]

   2. To provoke to combat or strife; to call out to combat; to
      challenge; to dare; to brave; to set at defiance; to treat
      with contempt; as, to defy an enemy; to defy the power of
      a magistrate; to defy the arguments of an opponent; to
      defy public opinion.
      [1913 Webster]

            I once again
            Defy thee to the trial of mortal fight. --Milton.
      [1913 Webster]

            I defy the enemies of our constitution to show the
            contrary.                             --Burke.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

defy \de*fy"\ (d[-e]*f[imac]"), n.
   A challenge. [Obs.] --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defy
    v 1: resist or confront with resistance; "The politician defied
         public opinion"; "The new material withstands even the
         greatest wear and tear"; "The bridge held" [syn: defy,
         withstand, hold, hold up]
    2: elude, especially in a baffling way; "This behavior defies
       explanation" [syn: defy, resist, refuse] [ant: apply,
       lend oneself]
    3: challenge; "I dare you!" [syn: defy, dare]

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "defy":
   affront, attack, baffle, balk, battle cry, be a man,
   be proof against, bear up, bear up against, beard, bid defiance,
   bid to combat, blast, brave, brazen out, break, break the law,
   breast, bring before, bring forward, bring up, call out,
   care naught for, cartel, challenge, checkmate, circumvent,
   confound, confront, confront with, contravene, counter, counteract,
   countermand, counterwork, cross, dare, dash, declare war, defeat,
   defi, deride, destroy, discomfit, disconcert, discountenance, dish,
   disobey, disregard, disrupt, do violence to, double dare,
   double-dare, elude, encounter, endure, envisage, face, face out,
   face with, flout, flummox, foil, front, frustrate, gage,
   gage of battle, gauntlet, gibe, glove, go counter to,
   have the guts, have the nerve, hold out, hold up, ignore, infringe,
   knock the chocks, lay before, levy war on, make bold to,
   make war on, meet, meet squarely, mock, nonplus, not conform,
   not heed, not keep, not listen, not mind, not observe,
   open hostilities, outdare, outface, perplex, place before,
   present to, put it to, rebel yell, rebuff, refuse to cooperate,
   repel, repulse, resist, ridicule, ruin, sabotage, scoff at, scorn,
   scotch, scout, scream defiance, set at defiance, set at naught,
   set before, set naught by, show fight, spike, spoil, stand,
   stand up, stand up to, stare down, stem, stonewall, stump, thwart,
   trample on, trample underfoot, trample upon, transgress, trespass,
   upset, venture, violate, war cry, war whoop, withstand