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):
fiendishly adv 1: as a devil; in an evil manner; "his writing could be diabolically satiric" [syn: diabolically, devilishly, fiendishly]