The Collaborative International Dictionary of English v.0.48:

Precarious \Pre*ca"ri*ous\, a. [L. precarius obtained by begging
   or prayer, depending on request or on the will of another,
   fr. precari to pray, beg. See Pray.]
   1. Depending on the will or pleasure of another; held by
      courtesy; liable to be changed or lost at the pleasure of
      another; as, precarious privileges. --Addison.
      [1913 Webster]

   2. Held by a doubtful tenure; depending on unknown causes or
      events; exposed to constant risk; not to be depended on
      for certainty or stability; uncertain; as, a precarious
      state of health; precarious fortunes. "Intervals of
      partial and precarious liberty." --Macaulay.
      [1913 Webster]

   Syn: Uncertain; unsettled; unsteady; doubtful; dubious;
        equivocal.

   Usage: Precarious, Uncertain. Precarious in stronger than
          uncertain. Derived originally from the Latin precari,
          it first signified "granted to entreaty," and, hence,
          "wholly dependent on the will of another." Thus it
          came to express the highest species of uncertainty,
          and is applied to such things as depend wholly on
          future casualties.
          [1913 Webster] -- Pre*ca"ri*ous*ly, adv. --
          Pre*ca"ri*ous*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

precarious
    adj 1: affording no ease or reassurance; "a precarious truce"
           [syn: precarious, unstable]
    2: 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]
    3: not secure; beset with difficulties; "a shaky marriage" [syn:
       precarious, shaky]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "precarious":
   ambiguous, borderline, chancy, critical, dangerous, delicate,
   desultory, dicey, difficult, doubtful, dubious, equivocal, hairy,
   hazardous, iffy, indecisive, infirm, insecure, insubstantial,
   on slippery ground, on thin ice, open, perilous, problematic,
   provisional, questionable, risky, sensitive, shaky, shifting,
   shifty, slippery, slippy, temporary, tentative, ticklish,
   touch-and-go, touchy, treacherous, tricky, uncertain, undependable,
   unfaithworthy, unpredictable, unreliable, unsettled, unsolid,
   unsound, unstable, unsteadfast, unsteady, unsubstantial, unsure,
   untrustworthy