The Collaborative International Dictionary of English v.0.48:

Impracticability \Im*prac`ti*ca*bil"i*ty\, n.; pl.
   Impracticabilities.
   1. The state or quality of being impracticable;
      infeasibility. --Goldsmith.
      [1913 Webster]

   2. An impracticable thing.
      [1913 Webster]

   3. Intractableness; stubbornness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impracticability
    n 1: the quality of not being usable [syn: impracticability,
         impracticableness] [ant: practicability,
         practicableness]