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]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "fiend":
   Apollyon, Baba Yaga, Beelzebub, LSD user, Lilith, Lucifer, Mafioso,
   Old Nick, Old Scratch, Satan, Young Turk, acidhead, addict,
   aficionado, afreet, alcoholic, ape-man, barghest, beast, beldam,
   berserk, berserker, bigot, bomber, brute, buff, bug, cacodemon,
   chain smoker, cocaine sniffer, cokie, collector, cubehead, daeva,
   demon, devil, devil incarnate, devotee, diablo, dipsomaniac,
   dope fiend, doper, dragon, drug abuser, drug addict, drug user,
   drunkard, dybbuk, eager beaver, energumen, enthusiast, evil genius,
   evil spirit, faddist, fan, fanatic, fiend from hell, fire-eater,
   firebrand, follower, freak, fury, genie, genius, ghoul,
   glue sniffer, goon, gorilla, great one for, gunsel, gyre, habitual,
   hardnose, harpy, head, heavy smoker, hell-raiser, hellcat,
   hellhound, hellion, hellkite, hobbyist, holy terror, hood, hoodlum,
   hophead, hothead, hotspur, hound, hype, incendiary, incubus,
   infatuate, jinni, jinniyeh, junkie, killer, lamia, mad dog, madcap,
   maniac, marijuana smoker, methhead, monster, mugger,
   narcotics addict, nut, ogre, ogress, pillhead, pothead, pursuer,
   rakshasa, rapist, revolutionary, rhapsodist, satan, savage,
   serpent, she-wolf, shedu, snowbird, speed freak, spitfire,
   succubus, sucker for, termagant, terror, terrorist, the undead,
   tiger, tigress, tough, tough guy, tripper, ugly customer, user,
   vampire, violent, virago, visionary, vixen, werewolf, wild beast,
   witch, wolf, yogini, zealot