buttocks vs tail end

buttocks and tail end both are nouns.

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

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

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

buttocks (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
	any projection that resembles the tail of an animal
	the time of the last part of something