offhanded vs impromptu

offhanded is not a noun while impromptu is a noun.

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

As adjectives, offhanded and impromptu are synonyms defined as:

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

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

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

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