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®

fiendishly
     adv : as a devil; in an evil manner; "his writing could be
           diabolically satiric" [syn: {diabolically}, {devilishly}]