The Collaborative International Dictionary of English v.0.48:

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; ad-lib;
   extempore; extemporary; as, an extemporaneous address or
   production. -- Ex*tem`po*ra"ne*ous*ly, adv. --
   Ex*tem`po*ra"ne*ous*ness,n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extemporaneously
    adv 1: without prior preparation; "he spoke extemporaneously"
           [syn: extemporaneously, extemporarily, extempore]