[adv] without prior preparation; "he spoke extemporaneously"
[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"; "offhand excuses"; "trying to sound offhanded and reassuring"; "an off-the-cuff toast"; "a few unrehearsed comments"
An improvised poem e.g. Extempore Effusion upon the Death of James Hogg by Wordsworth. See also impromptu.
ad-lib, extemporaneous, extemporaneously, extemporarily, extemporary, offhand, offhanded, off-the-cuff, unprepared, unrehearsed