Source : Webster's Revised Unabridged Dictionary (1913)

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; want
   of sufficient power, strength, resources, or capacity.

         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}.

Source : WordNet®

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