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"