posterior vs ass

posterior and ass both are nouns.

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

As nouns, posterior and ass are synonyms defined as:

posterior (noun)	ass (noun)
the fleshy part of the human body that you sit on	slang for sexual intercourse
a tooth situated at the back of the mouth	hardy and sure-footed animal smaller and with longer ears than the horse
	the fleshy part of the human body that you sit on
	a pompous fool

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