Entry 1 sense Webster, 1913 Fiendish /fēn'-dĭsh/ · Fiend·ish · IPA /ˈfiːndɪʃ/ 01 a. Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish. 1. Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish.