backside vs tail end

backside and tail end both are nouns.

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

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

backside and tail end: the fleshy part of the human body that you sit on

backside (noun)	tail end (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	any projection that resembles the tail of an animal
	the time of the last part of something