The Collaborative International Dictionary of English v.0.48:

Infeasibility \In*fea`si*bil"i*ty\, n.
   The state of being infeasible; impracticability.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infeasibility
    n 1: the quality of not being doable [syn: infeasibility,
         unfeasibility] [ant: feasibility, feasibleness]