extempore

adjectives

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"
ad-lib, extemporaneous, extemporary, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed

adverbs

without prior preparation
"he spoke extemporaneously"
extemporaneously, extemporarily, extempore