Synonyms for Extemporaneous (same or very similar meaning)
WordNet sense 1 (with little or no preparation or forethought):
ad-lib, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed, extemporary, offhandWordNet sense 2 (without preparation; not prepared for):
unpreparedFrom the ODE community, based on WordNetadd/edit