The Collaborative International Dictionary of English v.0.48:

Unworkable \Unworkable\
   See workable.

WordNet (r) 3.0 (2006):

unworkable
    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]