Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

fiendish
     adj : 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}, {hellish}, {infernal}, {satanic},
            {unholy}]