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]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "unworkable":
   beyond one, functionless, impracticable, impractical, in disrepair,
   inapplicable, infeasible, inoperable, inoperative, insuperable,
   insurmountable, nonfunctional, nonutilitarian, otiose,
   out of order, out of whack, too much for, unachievable,
   unattainable, uncompassable, unconducive, undoable, uneffectible,
   unemployable, unfeasible, unfit, unhelpful, unnegotiable,
   unperformable, unrealizable, unserviceable, unsuitable,
   unsurmountable, unusable, useless