keister vs fundament

keister and fundament both are nouns.

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

As nouns, keister and fundament are synonyms defined as:

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

keister (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