The Collaborative International Dictionary of English v.0.48:

Impracticableness \Im*prac"ti*ca*ble*ness\, n.
   The state or quality of being impracticable;
   impracticability.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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