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)]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "incapable":
   amputee, cripple, defective, deformity, disqualified, dull tool,
   greenhorn, handicapped person, idiot, ill-equipped, ill-fitted,
   ill-furnished, ill-provided, ill-qualified, imbecile,
   impervious to, inadequate, incapable of, incompetent, ineffective,
   ineffectual, inefficient, inept, inferior, maladjusted, mediocrity,
   no conjuror, not equal to, not up to, paralytic, paraplegic,
   quadriplegic, the crippled, the handicapped, unable, unable to,
   unadapted, unadjusted, unarmed, unendowed, unequal to, unequipped,
   unfit, unfitted, ungifted, unprovided, unqualified, unsuited,
   untalented