The Collaborative International Dictionary of English v.0.48:

Unfeasible \Unfeasible\
   See feasible.

WordNet (r) 3.0 (2006):

unfeasible
    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]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "unfeasible":
   beyond one, impracticable, impractical, infeasible, inoperable,
   insuperable, insurmountable, too much for, unachievable,
   unattainable, uncompassable, undoable, uneffectible, unnegotiable,
   unperformable, unrealizable, unsurmountable, unworkable