Definition of fiendishly adverb from the Oxford Advanced Learner's Dictionary

 

fiendishly

 adverb
adverb
BrE BrE//ˈfiːndɪʃli//
 
; NAmE NAmE//ˈfiːndɪʃli//
 
(informal)
 
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very; extremely fiendishly clever/complicated
See the Oxford Advanced American Dictionary entry: fiendishly