The Collaborative International Dictionary of English v.0.48:

Incapable \In*ca"pa*ble\, n.
   One who is morally or mentally weak or inefficient; an
   imbecile; a simpleton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Incapable \In*ca"pa*ble\, a. [Pref. in- not + capable: cf. F.
   incapable, L. incapabilis incomprehensible.]
   [1913 Webster]
   1. Lacking in ability or qualification for the purpose or end
      in view; not large enough to contain or hold; deficient in
      physical strength, mental or moral power, etc.; not
      capable; as, incapable of holding a certain quantity of
      liquid; incapable of endurance, of comprehension, of
      perseverance, of reform, etc.
      [1913 Webster]

   2. Not capable of being brought to do or perform, because
      morally strong or well disposed; -- used with reference to
      some evil; as, incapable of wrong, dishonesty, or
      falsehood.
      [1913 Webster]

   3. Not in a state to receive; not receptive; not susceptible;
      not able to admit; as, incapable of pain, or pleasure;
      incapable of stain or injury.
      [1913 Webster]

   4. (Law) Unqualified or disqualified, in a legal sense; as, a
      man under thirty-five years of age is incapable of holding
      the office of president of the United States; a person
      convicted on impeachment is thereby made incapable of
      holding an office of profit or honor under the government.
      [1913 Webster]

   5. (Mil.) As a term of disgrace, sometimes annexed to a
      sentence when an officer has been cashiered and rendered
      incapable of serving his country.
      [1913 Webster]

   Note: Incapable is often used elliptically.
         [1913 Webster]

               Is not your father grown incapable of reasonable
               affairs?                           --Shak.

   Syn: Incompetent; unfit; unable; insufficient; inadequate;
        deficient; disqualified. See Incompetent.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incapable
    adj 1: (followed by `of') lacking capacity or ability;
           "incapable of carrying a tune"; "he is incapable of
           understanding the matter"; "incapable of doing the work"
           [ant: capable]
    2: not being susceptible to or admitting of something (usually
       followed by `of'); "incapable of solution"
    3: (followed by `of') not having the temperament or inclination
       for; "simply incapable of lying" [ant: capable]
    4: not meeting requirements; "unequal to the demands put upon
       him" [syn: incapable, incompetent, unequal to(p)]