fundament vs backside

fundament and backside both are nouns.

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

As nouns, fundament and backside are synonyms defined as:

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

fundament (noun)	backside (noun)
lowest support of a structure	the fleshy part of the human body that you sit on
the fleshy part of the human body that you sit on	the side of an object that is opposite its front
the fundamental assumptions from which something is begun or developed or calculated or explained