[syn: demonic, diabolic, diabolical, fiendish, hellish, infernal, satanic, unholy]
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):
diabolical
    adj 1: 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" [syn: devilish, diabolic, diabolical,
           mephistophelian, mephistophelean]
    2: 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"; "infernal instruments of war";
       "satanic cruelty"; "unholy grimaces" [syn: demonic,
       diabolic, diabolical, fiendish, hellish, infernal,
       satanic, unholy]
Moby Thesaurus II by Grady Ward, 1.0:
61 Moby Thesaurus words for "diabolical":
   Draconian, Mephistophelian, Tartarean, animal, anthropophagous,
   atrocious, barbaric, barbarous, beastly, bestial, bloodthirsty,
   bloody, bloody-minded, brutal, brutalized, brute, brutish,
   cannibalistic, cruel, cruel-hearted, cursed, damnable, demoniac,
   demoniacal, demonic, demonish, demonlike, devil-like, devilish,
   diabolic, execrable, fell, feral, ferocious, fiendish, fiendlike,
   fierce, ghoulish, hellborn, hellish, infernal, inhuman, inhumane,
   murderous, ogreish, ruthless, sadistic, sanguinary, sanguineous,
   satanic, savage, sharkish, slavering, subhuman, truculent,
   unchristian, uncivilized, ungodly, unhuman, vicious, wolfish