The Collaborative International Dictionary of English v.0.48:

Inability \In`a*bil"i*ty\, n. [Pref. in- not + ability: cf. F.
   inhabilet['e]. See Able, and cf. Unable.]
   The quality or state of being unable; lack of ability; lack
   of sufficient power, strength, resources, or capacity.
   [1913 Webster]

         It is not from an inability to discover what they ought
         to do, that men err in practice.         --Blair.

   Syn: Impotence; incapacity; incompetence; weakness;
        powerlessness; incapability. See Disability.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inability
    n 1: lack of ability (especially mental ability) to do something
         [ant: ability, power]
    2: lacking the power to perform [syn: inability, unfitness]
       [ant: ability]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "inability":
   disability, disablement, disqualification, imbecility, inadeptness,
   inadequacy, inaptitude, inaptness, inattentiveness, incapability,
   incapacitation, incapacity, incompetence, incompetency,
   indexterity, ineffectiveness, ineffectuality, ineffectualness,
   inefficiency, ineptitude, ineptness, infancy, inferiority,
   insufficiency, legal incapacity, maladjustment, maladroitness,
   mediocrity, minority, pedestrianism, rustiness, skill-lessness,
   thoughtlessness, unaptness, uncleverness, undeftness,
   undexterousness, undextrousness, unfitness, unfittedness,
   unintelligence, unproficiency, unschooledness, unskillfulness,
   untrainedness, wardship