backside vs back end

backside and back end both are nouns.

Word	Noun	Adjective	Verb	Adverb
backside	Yes	No	No	No
back end	Yes	No	No	No

As nouns, backside and back end are synonyms defined as:

Other synonyms of backside include rear.

backside (noun)	back end (noun)
the fleshy part of the human body that you sit on	the side of an object that is opposite its front
the side of an object that is opposite its front