fundament vs rear end

fundament and rear end both are nouns.

Word	Noun	Adjective	Verb	Adverb
fundament	Yes	No	No	No
rear end	Yes	No	No	No

As nouns, fundament and rear end are synonyms defined as:

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

fundament (noun)	rear end (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 fundamental assumptions from which something is begun or developed or calculated or explained