tail end vs backside

tail end and backside both are nouns.

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

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

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

tail end (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
any projection that resembles the tail of an animal	the side of an object that is opposite its front
the time of the last part of something