Source : Webster's Revised Unabridged Dictionary (1913)

Fiend \Fiend\, n. [OE. fend, find, fiend, feond, fiend, foe, AS.
   fe['o]nd; akin to OS. f[=i]ond, D. vijand enemy, OHG.
   f[=i]ant, G. feind, Icel. fj[=a]nd, Sw. & Dan. fiende, Goth.
   fijands; orig. p. pr. of a verb meaning to hate, AS. fe['o]n,
   fe['o]gan, OHG. f[=i]?n, Goth. fijan, Skr. p[=i]y to scorn;
   prob. akin to E. feud a quarrel. [root]81. Cf. {Foe},
   {Friend}.]
   An implacable or malicious foe; one who is diabolically
   wicked or cruel; an infernal being; -- applied specifically
   to the devil or a demon.

         Into this wild abyss the wary fiend Stood on the brink
         of Hell and looked a while.              --Milton.

         O woman! woman! when to ill thy mind Is bent, all hell
         contains no fouler fiend.                --Pope.

Source : WordNet®

fiend
     n 1: a cruel wicked and inhuman person [syn: {monster}, {devil},
          {demon}, {ogre}]
     2: one of the evil spirits of traditional Jewish and Christian
        belief [syn: {devil}, {demon}, {daemon}, {daimon}]
     3: a person motivated by irrational enthusiasm (as for a
        cause); "A fanatic is one who can't change his mind and
        won't change the subject"--Winston Churchill [syn: {fanatic}]