The Collaborative International Dictionary of English v.0.48:

Furious \Fu"ri*ous\, a. [L. furiosus, fr. furia rage, fury: cf.
   F. furieux. See Fury.]
   1. Transported with passion or fury; raging; violent; as, a
      furious animal.
      [1913 Webster]

   2. Rushing with impetuosity; moving with violence; as, a
      furious stream; a furious wind or storm.

   Syn: Impetuous; vehement; boisterous; fierce; turbulent;
        tumultuous; angry; mad; frantic; frenzied. --
        Fu"ri*ous*ly, adv. -- Fu"ri*ous*ness, n.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

furiousness \fu"ri*ous*ness\ n.
   The quality of being furious.

   Syn: ferocity, fierceness, fury, vehemence, violence,
        wildness, strength.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

furiousness
    n 1: the property of being wild or turbulent; "the storm's
         violence" [syn: ferocity, fierceness, furiousness,
         fury, vehemence, violence, wildness]