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®

extemporaneously
     adv : without prior preparation; "he spoke extemporaneously" [syn:
            {extemporarily}, {extempore}]