Source : Webster's Revised Unabridged Dictionary (1913)

Extemporaneous \Ex*tem`po*ra"ne*ous\, a. [See {Extempore}.]
   Composed, performed, or uttered on the spur of the moment, or
   without previous study; unpremeditated; off-hand; extempore;
   extemporary; as, an extemporaneous address or production. --
   {Ex*tem`po*ra"ne*ous*ly}, adv. --
   {Ex*tem`po*ra"ne*ous*ness},n.

Source : WordNet®

extemporaneous
     adj : with little or no preparation or forethought; "his ad-lib
           comments showed poor judgment"; "an extemporaneous
           piano recital"; "an extemporary lecture"; "an extempore
           skit"; "an impromptu speech"; "offhand excuses";
           "trying to sound offhanded and reassuring"; "an
           off-the-cuff toast"; "a few unrehearsed comments" [syn:
            {ad-lib}, {extemporary}, {extempore}, {impromptu}, {offhand},
            {offhanded}, {off-the-cuff}, {unrehearsed}]