posterior vs keister

posterior and keister both are nouns.

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

As nouns, posterior and keister are synonyms defined as:

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

posterior (noun)	keister (noun)
the fleshy part of the human body that you sit on	the fleshy part of the human body that you sit on
a tooth situated at the back of the mouth

posterior (adjective)	keister (adjective)
located at or near or behind a part or near the end of a structure
coming at a subsequent time or stage