impromptu vs off-the-cuff

impromptu is a noun but off-the-cuff is not a noun.

Word	Noun	Adjective	Verb	Adverb
impromptu	Yes	Yes	No	Yes
off-the-cuff	No	Yes	No	No

As adjectives, impromptu and off-the-cuff are synonyms defined as:

impromptu and off-the-cuff: with little or no preparation or forethought

Other synonyms of impromptu include ad-lib, extemporaneous, extemporary, extempore, offhand, offhanded, unrehearsed.

impromptu (noun)	off-the-cuff (noun)
an extemporaneous speech or remark
a short musical passage that seems to have been made spontaneously without advance preparation

impromptu (adjective)	off-the-cuff (adjective)
with little or no preparation or forethought	with little or no preparation or forethought

impromptu (adverb)	off-the-cuff (adverb)
without advance preparation