Impromptu

noun
a short musical passage that seems to have been made spontaneously without advance preparation
adjective
with little or no preparation or forethought
“his ad-lib comments showed poor judgment”
Similar: extemporaneous, extemporary, extempore, offhand, offhanded, unrehearsed
noun
an extemporaneous speech or remark
“a witty impromptu must not sound premeditated”
adverb
without advance preparation
“he spoke ad lib”
Similar: ad lib, ad libitum, spontaneously