Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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. -- {Pre*ca"ri*ous*ly},
          adv. -- {Pre*ca"ri*ous*ness}, n.

Source : WordNet®

precariousness
     n : extreme dangerousness