ass vs posterior

ass and posterior both are nouns.

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

As nouns, ass and posterior are synonyms defined as:

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

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