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):

perilous
    adj 1: fraught with danger; "dangerous waters"; "a parlous
           journey on stormy seas"; "a perilous voyage across the
           Atlantic in a small boat"; "the precarious life of an
           undersea diver"; "dangerous surgery followed by a touch-
           and-go recovery" [syn: parlous, perilous,
           precarious, touch-and-go]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "perilous":
   alarming, bad, chancy, critical, dangerous, dangersome, delicate,
   desultory, explosive, fraught with danger, hairy, hazardous,
   infirm, insecure, insubstantial, jeopardous, menacing, parlous,
   periculous, precarious, provisional, risky, serious, shaky,
   shifting, shifty, slippery, susceptible, temporary, tentative,
   threatening, ticklish, tottery, touchy, treacherous, ugly,
   uncertain, undependable, unfaithworthy, unhealthy, unreliable,
   unsafe, unsolid, unsound, unstable, unsteadfast, unsteady,
   unsubstantial, unsure, untrustworthy, vulnerable, wicked