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.]
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            I defy the surety and the bond.       --Chaucer.
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            For thee I have defied my constant mistress. --Beau.
                                                  & Fl.
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   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.
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            I once again
            Defy thee to the trial of mortal fight. --Milton.
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            I defy the enemies of our constitution to show the
            contrary.                             --Burke.
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The Collaborative International Dictionary of English v.0.48:

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