Source : Webster's Revised Unabridged Dictionary (1913)

Perilous \Per"il*ous\, a. [OF. perillous, perilleus, F.
   p['e]rilleux, L. periculosus. See {Peril}.] [Written also
   {perillous}.]
   1. Full of, attended with, or involving, peril; dangerous;
      hazardous; as, a perilous undertaking.

            Infamous hills, and sandy, perilous wilds. --Milton.

   2. Daring; reckless; dangerous. [Obs.] --Latimer.

            For I am perilous with knife in hand. --Chaucer.
      -- {Per"il*ous*ly}, adv. -- {Per"il*ous*ness}, n.

Source : WordNet®

perilousness
     n : the state of being dangerous [syn: {hazardousness}]