/ ˈfiːndɪʃ; NAmE / adjective [ usually before noun ]
1.
cruel and unpleasant :
a fiendish act
shrieks of fiendish laughter
2.
( informal ) extremely clever and complicated, often in an unpleasant way :
a puzzle of fiendish complexity
a fiendish plan
3.
( informal ) extremely difficult :
a fiendish problem