The Collaborative International Dictionary of English v.0.48:

Incapacity \In`ca*pac"i*ty\, n.; pl. Incapacities. [Cf. F.
   incapacit['e].]
   [1913 Webster]
   1. Lack of capacity; lack of physical or intellectual power;
      inability.
      [1913 Webster]

   2. (Law) Lack of legal ability or competency to do, give,
      transmit, or receive something; inability;
      disqualification; as, the inacapacity of minors to make
      binding contracts, etc.

   Syn: Inability; incapability; incompetency; unfitness;
        disqualification; disability.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incapacity
    n 1: lack of intellectual power [ant: capacity, mental
         ability]
    2: lack of physical or natural qualifications [ant: capacity]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "incapacity":
   Urdummheit, brainlessness, caducity, decrepitude, disability,
   disablement, disqualification, foolishness, ignorance, imbecility,
   inability, inadeptness, inadequacy, inaptitude, inaptness,
   inattentiveness, incapability, incapacitation, incompetence,
   incompetency, indexterity, ineffectiveness, ineffectuality,
   inefficacy, inefficiency, ineptitude, ineptness, infancy,
   inferiority, infirmity, insipidity, instability, insufficiency,
   invalidism, irrationality, lackbrainedness, lackwittedness,
   legal incapacity, low IQ, maladjustment, maladroitness, mediocrity,
   mental weakness, mindlessness, minority, pedestrianism,
   primal stupidity, reasonlessness, ricketiness, rustiness, senility,
   senselessness, shakiness, skill-lessness, slackmindedness,
   slackwittedness, thoughtlessness, unaptness, uncleverness,
   undeftness, undexterousness, undextrousness, unfirmness, unfitness,
   unfittedness, unintellectuality, unintelligence, unproficiency,
   unschooledness, unskillfulness, unsoundness, unsteadiness,
   unsturdiness, unsubstantiality, untrainedness, unwisdom,
   unwiseness, vapidity, wardship, wateriness, wishy-washiness,
   witlessness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INCAPACITY. The want of a quality legally to do, give, transmit, or receive
something.
     2. It arises from nature, from the law, or from both. From nature, when
the party has not his senses, as, in the case of an idiot; from the law, as,
in the case of a bastard who cannot inherit from nature and the law; as, in
the case of a married woman, who cannot make contracts or a will.
     3. In general, the incapacity ceases with the cause which produces it.
If the idiot should obtain his senses, or the married woman's husband die,
their incapacity would be at an end.
     4. When a cause of action arises during the incapacity of a person
having the right to sue, the act of limitation does not, in general,
commence to run till the incapacity has been removed. But two incapacities
cannot be joined in order to come within the statute.