The Collaborative International Dictionary of English v.0.48:

Diabolic \Di`a*bol"ic\, Diabolical \Di`a*bol"ic*al\, a. [L.
   diabolicus, Gr. ? 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.

WordNet (r) 3.0 (2006):

diabolically
    adv 1: as a devil; in an evil manner; "his writing could be
           diabolically satiric" [syn: diabolically, devilishly,
           fiendishly]