tail end vs buttocks

tail end and buttocks both are nouns.

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

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

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

tail end (noun)	buttocks (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 time of the last part of something