ad-lib vs unprepared

ad-lib is a noun but unprepared is not a noun.

Word	Noun	Adjective	Verb	Adverb
ad-lib	Yes	Yes	Yes	No
unprepared	No	Yes	No	No

ad-lib (noun)	unprepared (noun)
remark made spontaneously without prior preparation

ad-lib (adjective)	unprepared (adjective)
with little or no preparation or forethought	without preparation; not prepared for
said or done without having been planned or written in advance

ad-lib (verb)	unprepared (verb)
perform without preparation