unrealizable

from WordNet (r) 3.0 (2006)

unrealizable
    adj 1: impossible to achieve; "an unattainable goal" [syn:
           {unachievable}, {unattainable}, {undoable},
           {unrealizable}]

from Moby Thesaurus II by Grady Ward, 1.0

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