[1] : The Collaborative International Dictionary of English v.0.48

diabolic \di`abol"ic\, diabolical \di`abol"ical\, 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`abol"ically,
        adv. -- di`abol"icalness, n.
see also:
devil di`abol"ically di`abol"icalness 

[2] : WordNet (r) 2.0

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, 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,
         fiendish, hellish, infernal, satanic, unholy
see also:
devilish diabolic mephistophelian mephistophelean demonic fiendish 
hellish infernal satanic unholy 

[3] : 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

Diabolical >> D Words
Diabolical, definition of term: Diabolical
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