The Collaborative International Dictionary of English v.0.48:

Fiendish \Fiend"ish\, a.
   Like a fiend; diabolically wicked or cruel; infernal;
   malignant; devilish; hellish. -- Fiend"ish*ly, adv. --
   Fiend"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fiendish
    adj 1: extremely evil or cruel; expressive of cruelty or
           befitting hell; "something demonic in him--something that
           could be cruel"; "fires lit up a diabolic scene";
           "diabolical sorcerers under the influence of devils"; "a
           fiendish despot"; "hellish torture"; "infernal
           instruments of war"; "satanic cruelty"; "unholy grimaces"
           [syn: demonic, diabolic, diabolical, fiendish,
           hellish, infernal, satanic, unholy]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "fiendish":
   Draconian, Mephistophelian, Tartarean, animal, anthropophagous,
   atrocious, bad, baleful, barbaric, barbarous, beastly, bestial,
   bloodthirsty, bloody, bloody-minded, brutal, brutalized, brute,
   brutish, cannibalistic, cruel, cruel-hearted, cursed, damnable,
   demoniac, demoniacal, demonic, demonish, demonlike, devil-like,
   devilish, diabolic, diabolical, evil, execrable, fell, feral,
   ferocious, fiendlike, fierce, ghoulish, heinous, hellborn, hellish,
   infernal, inhuman, inhumane, malefic, maleficent, malevolent,
   malicious, malign, malignant, monstrous, murderous, ogreish,
   outrageous, ruthless, sadistic, sanguinary, sanguineous, satanic,
   savage, serpentine, sharkish, sinister, slavering, subhuman,
   truculent, unchristian, uncivilized, ungodly, unhallowed, unhuman,
   vicious, wicked, wolfish