offhand vs impromptu

offhand is not a noun while impromptu is a noun.

Word	Noun	Adjective	Verb	Adverb
offhand	No	Yes	No	Yes
impromptu	Yes	Yes	No	Yes

As adjectives, offhand and impromptu are synonyms defined as:

Other synonyms of offhand include ad-lib, extemporaneous, extemporary, extempore, off-the-cuff, offhanded, unrehearsed.

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

offhand (adjective)	impromptu (adjective)
casually thoughtless or inconsiderate	with little or no preparation or forethought
with little or no preparation or forethought

offhand (adverb)	impromptu (adverb)
in a casually inconsiderate manner	without advance preparation
without previous thought or preparation