The Collaborative International Dictionary of English v.0.48:

Infeasible \In*fea"si*ble\, a.
   Not capable of being done or accomplished; impracticable.
   --Glanvill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infeasible
    adj 1: not capable of being carried out or put into practice;
           "refloating the sunken ship proved impracticable because
           of its fragility"; "a suggested reform that was
           unfeasible in the prevailing circumstances" [syn:
           impracticable, infeasible, unfeasible,
           unworkable]