ad-lib

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"

said or done without having been planned or written in advance; "he made a few ad-lib remarks"

remark made spontaneously without prior preparation; "his ad-libs got him in trouble with the politicians"

perform without preparation; "he extemporized a speech at the wedding"