impromptu

an extemporaneous speech or remark; "a witty impromptu must not sound premeditated"

a short musical passage that seems to have been made spontaneously without advance preparation

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"

without advance preparation; "he spoke ad lib"