[1] : The Collaborative International Dictionary of English v.0.48

inability \in`abil"ity\, 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
see also:
able unable disability 

[2] : WordNet (r) 2.0

inability
     n 1: lack of ability especially mental ability to do something
          ant: ability
     2: lacking the power to perform syn: unfitness ant: ability
see also:
ability unfitness 

[3] : 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

Inability >> I Words
Inability, definition of term: Inability
inability_pag1.html