bum vs backside

bum and backside both are nouns.

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

As nouns, bum and backside are synonyms defined as:

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

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

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