buttocks vs fundament

buttocks and fundament both are nouns.

Word	Noun	Adjective	Verb	Adverb
buttocks	Yes	No	No	No
fundament	Yes	No	No	No

As nouns, buttocks and fundament are synonyms defined as:

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

buttocks (noun)	fundament (noun)
the fleshy part of the human body that you sit on	lowest support of a structure
	the fleshy part of the human body that you sit on
	the fundamental assumptions from which something is begun or developed or calculated or explained