The Collaborative International Dictionary of English v.0.48:

Prone \Prone\, a. [L. pronus, akin to Gr. ?, ?, Skr. pravana
   sloping, inclined, and also to L. pro forward, for. See
   Pro-.]
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   1. Bending forward; inclined; not erect.
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            Towards him they bend
            With awful reverence prone.           --Milton.
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   2. Prostrate; flat; esp., lying with the face down; --
      opposed to supine.
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            Which, as the wind,
            Blew where it listed, laying all things prone.
                                                  --Byron.
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   3. Headlong; running downward or headlong. "Down thither
      prone in flight." --Milton.
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   4. Sloping, with reference to a line or surface; declivous;
      inclined; not level.
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            Since the floods demand,
            For their descent, a prone and sinking land.
                                                  --Blackmore.
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   5. Inclined; propense; disposed; -- applied to the mind or
      affections, usually in an ill sense. Followed by to.
      "Prone to mischief." --Shak.
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            Poets are nearly all prone to melancholy. --Landor.
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WordNet (r) 3.0 (2006):

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