bum vs arse around

bum is a noun but arse around is not a noun.

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

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

bum (noun)	arse around (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

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

bum (verb)	arse around (verb)
ask for and get free; be a parasite	indulge in horseplay
be lazy or idle	be lazy or idle