piece of ass vs coitus

piece of ass and coitus both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of ass	Yes	No	No	No
coitus	Yes	No	No	No

As nouns, coitus is a hypernym of piece of ass; that is, coitus is a word with a broader meaning than piece of ass:

piece of ass: slang for sexual intercourse
coitus: sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur

piece of ass (noun)	coitus (noun)
slang for sexual intercourse	sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur