The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Infamous hills, and sandy, perilous wilds. --Milton.
      [1913 Webster]

   2. Daring; reckless; dangerous. [Obs.] --Latimer.
      [1913 Webster]

            For I am perilous with knife in hand. --Chaucer.
      [1913 Webster] -- Per"il*ous*ly, adv. --
      Per"il*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

perilously
    adv 1: in a dangerous manner; "he came dangerously close to
           falling off the ledge" [syn: perilously, hazardously,
           dangerously]