Source : Webster's Revised Unabridged Dictionary (1913)

Prone \Prone\, a. [L. pronus, akin to Gr. ?, ?, Skr. pravana
   sloping, inclined, and also to L. pro forward, for. See
   {Pro-}.]
   1. Bending forward; inclined; not erect.

            Towards him they bend With awful reverence prone.
                                                  --Milton.

   2. Prostrate; flat; esp., lying with the face down; --
      opposed to {supine}.

            Which, as the wind, Blew where it listed, laying all
            things prone.                         --Byron.

   3. Headlong; running downward or headlong. ``Down thither
      prone in flight.'' --Milton.

   4. Sloping, with reference to a line or surface; declivous;
      inclined; not level.

            Since the floods demand, For their descent, a prone
            and sinking land.                     --Blackmore.

   5. Inclined; propense; disposed; -- applied to the mind or
      affections, usually in an ill sense. Followed by to.
      ``Prone to mischief.'' --Shak.

            Poets are nearly all prone to melancholy. --Landor.

Source : WordNet®

prone
     adj 1: lying face downward [syn: {prostrate}]
     2: having a tendency (to); often used in combination; "a child
        prone to mischief"; "failure-prone"