The Collaborative International Dictionary of English v.0.48:

Impossible \Im*pos"si*ble\, n.
   An impossibility; as, he tried to do the impossible. [Obs.]
   [1913 Webster]

         "Madam," quoth he, "this were an impossible!"
                                                  --Chaucer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impossible \Im*pos"si*ble\, a. [F., fr. L. impossibilis; pref.
   im- not + possibilis possible. See Possible.]
   Not possible; incapable of being done, of existing, etc.;
   unattainable in the nature of things, or by means at command;
   insuperably difficult under the circumstances; absurd or
   impracticable; not feasible.
   [1913 Webster]

         With men this is impossible; but with God all things
         are possible.                            --Matt. xix.
                                                  26.
   [1913 Webster]

         Without faith it is impossible to please him. --Heb.
                                                  xi. 6.
   [1913 Webster]

   Impossible quantity (Math.), an imaginary quantity. See
      Imaginary.

   Syn: See Impracticable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impossible
    adj 1: not capable of occurring or being accomplished or dealt
           with; "an impossible dream"; "an impossible situation"
           [ant: possible]
    2: totally unlikely [syn: impossible, inconceivable, out of
       the question, unimaginable]
    3: used of persons or their behavior; "impossible behavior";
       "insufferable insolence" [syn: impossible, insufferable,
       unacceptable, unsufferable]
    n 1: something that cannot be done; "his assignment verged on
         the impossible"

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "impossible":
   I refuse, I will not, absurd, algorismic, algorithmic, aliquot,
   antic, barred, by no means, cardinal, closed-out,
   contrary to reason, count me out, crazy, cureless, decimal,
   differential, digital, even, exceptionable, excluded, exponential,
   fanciful, fantastic, farcical, figural, figurate, figurative,
   finite, fractional, hopeless, illogical, imaginary, immedicable,
   impair, impracticable, impractical, inadmissible, include me out,
   incomprehensible, inconceivable, incredible, incurable,
   indefensible, infeasible, infinite, integral, intolerable,
   irrational, irremediable, irreparable, logarithmic,
   logically impossible, logometric, ludicrous, negative, no, no go,
   not likely, not possible, nothing doing, numeral, numerary,
   numerative, numeric, objectionable, odd, ordinal, outlandish,
   outrageous, oxymoronic, pair, paradoxical, positive, possible,
   preposterous, prime, prohibited, radical, rational, real,
   reciprocal, ridiculous, ruled-out, self-contradictory, submultiple,
   surd, transcendental, unacceptable, unaccountable, unachievable,
   unattainable, unbelievable, uncorrectable, undesirable, unexpected,
   unfeasible, unimaginable, unobtainable, unrealizable, unreasonable,
   unsolvable, unsuitable, untenable, unthinkable, unworkable,
   weird