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):

extemporaneous
    adj 1: 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,
           extemporaneous, extemporary, extempore,
           impromptu, offhand, offhanded, off-the-cuff,
           unrehearsed]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "extemporaneous":
   ad lib, ad libitum, ad-lib, casual, caught napping,
   caught off balance, caught short, extemporary, extempore,
   extemporized, haphazard, hasty, impromptu, improvisatorial,
   improvise, improvised, improviso, impulsive, informal, jury-rigged,
   makeshift, off the cuff, off-the-cuff, offhand, precipitate,
   rough-and-ready, snap, spontaneous, spur-of-the-moment, stopgap,
   surprised, taken aback, taken by surprise, taken unawares,
   tripped up, unarranged, unbegun, unconcocted, uncontrived,
   undeliberated, undevised, unhatched, unmade, unmanufactured,
   unorganized, unplanned, unpremeditated, unprepared, unprimed,
   unready, unrehearsed, unstudied