hard up vs poor

hard up is not a noun while poor is a noun.

Word	Noun	Adjective	Verb	Adverb
hard up	No	Yes	No	No
poor	Yes	Yes	No	No

hard up (noun)	poor (noun)
	people without possessions or wealth (considered as a group)

hard up (adjective)	poor (adjective)
not having enough money to pay for necessities	having little money or few possessions
	lacking in quality or substances
	characterized by or indicating poverty
	of insufficient quantity to meet a need
	deserving or inciting pity