#1 diabolical s. [devilish, diabolic, mephistophelian, mephistophelean] showing the cunning or ingenuity or wickedness typical of a devil; "devilish schemes"; "the cold calculation and diabolic art of some statesmen"; "the diabolical expression on his face"; "a mephistophelian glint in his eye". wn
#2 diabolical s. [demonic, diabolic, fiendish, hellish, infernal, satanic, unholy] extremely evil or cruel; expressive of cruelty or befitting hell; "something demonic in him--something that could be cruel"; "fires lit up a diabolic scene"; "diabolical sorcerers under the influence of devils"; "a fiendish despot"; "hellish torture"; "in. wn
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