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Definition of fiendish : Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish.

Synonyms:

damn, satanical, beastly, blasted, ghoulish, goddamn, mephistophelian, darned, infernal, god-awful, unholy, blessed, goddamned, damned, blame, sinful, mephistophelean, diabolical, evil, wicked, unhallowed, deuced, kind, blamed, goddam, ogreish

neurotic, lunatic, nutty, psychotic, kooky, mad, crazy, crackpot

devilish, demonic, satanic, diabolic

dog-eat-dog, pitiless, stony, brutal, inhumane, unsympathetic, unfeeling, cruel, bloodthirsty, ruthless, unpitying, cutthroat, hardhearted, barbaric, demoniac, unforgiving, uncompassionate, unmerciful, merciless, hellish

Usage examples:

• No one seemed to notice him, though several times he caught Evil- Eye regarding him with such a look of fiendish triumph as sent a shiver to his heart. - "The Wreckers of Sable Island", J. Macdonald Oxley.
• As the ragged mob of men, women and children, with hollow cheeks and pale faces threw themselves at his feet crying for bread, a still more fiendish plan suggested itself. - "Legends of the Rhine", Wilhelm Ruland.
• A man having ventured to say that he thought that the Republic might be established without shedding the blood of Louis, was immediately stabbed to the heart, and his mutilated remains were dragged through the streets of Paris in fiendish revelry. - "Madame Roland, Makers of History", John S. C. Abbott.