backside vs bum

backside and bum both are nouns.

Word	Noun	Adjective	Verb	Adverb
backside	Yes	No	No	No
bum	Yes	Yes	Yes	No

As nouns, backside and bum are synonyms defined as:

backside (noun)	bum (noun)
the fleshy part of the human body that you sit on	the fleshy part of the human body that you sit on
the side of an object that is opposite its front	a vagrant
	a person who is deemed to be despicable or contemptible
	person who does no work

backside (adjective)	bum (adjective)
	of very poor quality; flimsy

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