The Collaborative International Dictionary of English v.0.48:

fiend \fiend\ (f[=e]nd), n. [OE. fend, find, fiend, feond,
   fiend, foe, AS. fe['o]nd; akin to OS. f[imac]ond, D. vijand
   enemy, OHG. f[imac]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[imac]?n, Goth. fijan,
   Skr. p[imac]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.
   [1913 Webster]

         Into this wild abyss the wary fiend
         Stood on the brink of Hell and looked a while.
                                                  --Milton.
   [1913 Webster]

         O woman! woman! when to ill thy mind
         Is bent, all hell contains no fouler fiend. --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fiend
    n 1: a cruel wicked and inhuman person [syn: monster, fiend,
         devil, demon, ogre]
    2: an evil supernatural being [syn: devil, fiend, 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, fiend]