{ } a. [ L. diabolicus, Gr. &unr_; devilish, slanderous: cf. F. diabolique. See Devil. ] 1. Pertaining to the devil; resembling, or appropriate, or appropriate to, the devil; befitting hell or satan; devilish; infernal; impious; as, a diabolic or diabolical temper or act; the diabolical expression on his face; fires lit up a diabolic scene. “Diabolic power.” Milton. “The diabolical institution.” Motley.
Syn. -- devilish, mephistophelian, mephistophelean. [ 1913 Webster ]

2. showing a wicked cunning or ingenuity; as, the cold calculation and diabolic art of some statesmen.
Syn. -- devilish, mephistophelian, mephistophelean. [ WordNet 1.5 ]

3. extremely evil or cruel; atrocious; outrageously wicked; as, diabolical sorcerers under the influence of devils; diabolical torturers taking pleasure in their craft.
Syn. -- demonic, fiendish, hellish, infernal, nefarious, satanic. [ 1913 Webster + WordNet 1.5 ]

-- Di`a*bol"ic*al*ly, adv. -- Di`a*bol"ic*al*ness, n.