backside vs fundament

backside and fundament both are nouns.

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

As nouns, backside and fundament are synonyms defined as:

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

backside (noun)	fundament (noun)
the fleshy part of the human body that you sit on	lowest support of a structure
the side of an object that is opposite its front	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