impromptu vs offhand

impromptu is a noun but offhand is not a noun.

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

As adjectives, impromptu and offhand are synonyms defined as:

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

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

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

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