hard up vs penniless

hard up and penniless both are adjectives.

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

As adjectives, hard up and penniless are synonyms defined as:

Other synonyms of hard up include impecunious, in straitened circumstances, penurious, pinched.

hard up (adjective)	penniless (adjective)
not having enough money to pay for necessities	not having enough money to pay for necessities