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.
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   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]