rear end vs fundament

rear end and fundament both are nouns.

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

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

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

rear end (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