[syn: blasted, blame, blamed, blessed, damn, damned, darned, deuced, goddam, goddamn, goddamned, infernal]
4.  of or pertaining to or characteristic of a very uncontrolled and intense fire; 
- Example: "infernal heat"
5.  being of the underworld; 
- Example: "infernal regions"
The Collaborative International Dictionary of English v.0.48:
Infernal \In*fer"nal\, n.
   An inhabitant of the infernal regions; also, the place
   itself. [Obs.] --Drayton.
   [1913 Webster]
The Collaborative International Dictionary of English v.0.48:
Infernal \In*fer"nal\, a. [F. infernal, L. infernalis, fr.
   infernus that which lies beneath, the lower. See Inferior.]
   [1913 Webster]
   1. Of or pertaining to or suitable for the lower regions,
      inhabited, according to the ancients, by the dead;
      pertaining to Pluto's realm of the dead, the Tartarus of
      the ancients.
      [1913 Webster]
            The Elysian fields, the infernal monarchy. --Garth.
      [1913 Webster]
   2. Of or pertaining to, resembling, or inhabiting, hell;
      suitable for hell, or to the character of the inhabitants
      of hell; hellish; diabolical; as, infernal spirits, or
      conduct.
      [1913 Webster]
            The instruments or abettors in such infernal
            dealings.                             --Addison.
      [1913 Webster]
   Infernal machine, a machine or apparatus maliciously
      designed to explode, and destroy life or property.
   Infernal stone (lapis infernalis), lunar caustic;
      formerly so called. The name was also applied to caustic
      potash.
   Syn: Tartarean; Stygian; hellish; devilish; diabolical;
        satanic; fiendish; malicious.
        [1913 Webster]
WordNet (r) 3.0 (2006):
infernal
    adj 1: characteristic of or resembling Hell; "infernal noise";
           "infernal punishment"
    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]
    3: expletives used informally as intensifiers; "he's a blasted
       idiot"; "it's a blamed shame"; "a blame cold winter"; "not a
       blessed dime"; "I'll be damned (or blessed or darned or
       goddamned) if I'll do any such thing"; "he's a damn (or
       goddam or goddamned) fool"; "a deuced idiot"; "an infernal
       nuisance" [syn: blasted, blame, blamed, blessed,
       damn, damned, darned, deuced, goddam, goddamn,
       goddamned, infernal]
    4: of or pertaining to or characteristic of a very uncontrolled
       and intense fire; "infernal heat"
    5: being of the underworld; "infernal regions" [ant: supernal]
    n 1: an inhabitant of Hell; "his roar made the infernals quake"
Moby Thesaurus II by Grady Ward, 1.0:
100 Moby Thesaurus words for "infernal":
   Acherontic, Draconian, Hadean, Lethean, Mephistophelian, Plutonian,
   Plutonic, Stygian, Tartarean, absolute, animal, anthropophagous,
   atrocious, barbaric, barbarous, beastly, bestial, blamed, blasted,
   blessed, bloodthirsty, bloody, bloody-minded, brutal, brutalized,
   brute, brutish, cannibalistic, chthonian, chthonic, cimmerian,
   confounded, cruel, cruel-hearted, cursed, cussed, damnable, damned,
   demoniac, demoniacal, demonic, demonical, devilish, diabolic,
   diabolical, dire, downright, dreadful, evil, execrable, fell,
   feral, ferocious, fiendish, fiendlike, fierce, flagitious,
   ghoulish, heinous, hellborn, hellish, inhuman, inhumane,
   iniquitous, maleficent, malevolent, malicious, murderous, nether,
   out-and-out, outright, pandemoniac, pandemonic, plutonian,
   plutonic, purgatorial, ruthless, sadistic, sanguinary, sanguineous,
   satanic, savage, sharkish, sinister, slavering, straight-out,
   stygian, subhuman, sulfurous, thoroughgoing, truculent,
   unchristian, uncivilized, underworld, ungodly, unhuman, vicious,
   villainous, wicked, wolfish