diabolical UK [ˌdaɪəˈbɒlɪkəɫ]US [ˌdaɪəˈbɑlɪkəɫ] adjective4 •Of or concerning the devil: devilish, satanic. •Extremely wicked or cruel. •(slang) (Devilishly) cunning or devious. •(slang) Crazy, wild (sometimes especially due to being unexpected or surprising). formsmore diabolical · most diabolical