The Collaborative International Dictionary of English v.0.48:

Impracticable \Im*prac"ti*ca*ble\, a.
   1. Not practicable; incapable of being performed, or
      accomplished by the means employed, or at command;
      impossible; as, an impracticable undertaking.
      [1913 Webster]

   2. Not to be overcome, persuaded, or controlled by any
      reasonable method; unmanageable; intractable; not capable
      of being easily dealt with; -- used in a general sense, as
      applied to a person or thing that is difficult to control
      or get along with.
      [1913 Webster]

            This though, impracticable heart
            Is governed by a dainty-fingered girl. --Rowe.
      [1913 Webster]

            Patriotic but loyal men went away disgusted afresh
            with the impracticable arrogance of a sovereign.
                                                  --Palfrey.
      [1913 Webster]

   3. Incapable of being used or availed of; as, an
      impracticable road; an impracticable method.

   Syn: Impossible; infeasible. -- Impracticable,
        Impossible. A thing is impracticable when it can not
        be accomplished by any human means at present possessed;
        a thing is impossible when the laws of nature forbid it.
        The navigation of a river may now be impracticable, but
        not impossible, because the existing obstructions may
        yet be removed. "The barons exercised the most despotic
        authority over their vassals, and every scheme of public
        utility was rendered impracticable by their continued
        petty wars with each other." --Mickle. "With men this is
        impossible, but with God all things are possible."
        --Matt. xix. 26.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impracticable
    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:

22 Moby Thesaurus words for "impracticable":
   awkward, disadvantageous, impossible, impractical, inapplicable,
   inconvenient, infeasible, nonfunctional, troublesome, unacceptable,
   unachievable, unattainable, undesirable, unfeasible, unfit,
   unrealizable, unsatisfactory, unserviceable, unsuitable, unusable,
   unworkable, useless