extemporaneous
definition
done without preparation or planning
example
During the debate, she delivered an extemporaneous speech that captivated the audience and showed her quick thinking.
synonyms
improvisedspontaneousunprepared
word origin — from Latin 'extemporaneus', meaning 'out of the moment' or 'offhand', derived from 'ex-' (out of) + 'tempus' (time)