arse about vs bum

arse about is not a noun while bum is a noun.

Word	Noun	Adjective	Verb	Adverb
arse about	No	No	Yes	No
bum	Yes	Yes	Yes	No

As verbs, arse about and bum are synonyms defined as:

arse about (noun)	bum (noun)
	the fleshy part of the human body that you sit on
	a vagrant
	a person who is deemed to be despicable or contemptible
	person who does no work

arse about (adjective)	bum (adjective)
	of very poor quality; flimsy

arse about (verb)	bum (verb)
be lazy or idle	ask for and get free; be a parasite
	be lazy or idle