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-.]
   [1913 Webster]
   1. Bending forward; inclined; not erect.
      [1913 Webster]

            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.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "prone":
   accumbent, acquiescent, agreeable, alacritous, amenable,
   answerable for, apt, apt to, ardent, bent, calculated to,
   capable of, compliant, consenting, content, cooperative, couchant,
   couche, crawling, crouched, crouching, debased, decumbent,
   deferential, dependent on, depressed, disposed, disposed to,
   dispositioned, docile, draped, eager, enthusiastic, exposed,
   exposed to, fain, favorable, favorably disposed,
   favorably inclined, flat, forward, game, given, given to,
   groveling, horizontal, in danger of, in the mind, in the mood,
   incident to, inclined, inclined to, knee-high, knocked flat,
   laid low, leaning, level, liable, liable to, likely, likely to,
   lolling, lounging, low, low-built, low-hung, low-level,
   low-leveled, low-lying, low-set, low-statured, lying, lying down,
   minded, minded to, naked to, neap, obeisant, obliged to, obnoxious,
   obsequious, on bended knee, open, open to, pliant, predisposed,
   predisposed to, procumbent, prompt, prone to, prostrate, quick,
   ready, ready and willing, ready for, ready to, receptive,
   reclining, recumbent, reposing, responsible for, responsive,
   resupine, runty, sensitive, servile, short, sprawled, sprawling,
   spread, squat, squatty, standing to, stooped, stumpy, subject,
   subject to, subservient, supine, susceptible, susceptive to,
   tending, tractable, unelevated, well-disposed, well-inclined,
   willed, willing, willinghearted, within range of, zealous