coitus vs piece of ass

coitus and piece of ass both are nouns.

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

As nouns, piece of ass is a hyponym of coitus; that is, piece of ass is a word with a more specific, narrower meaning than coitus:

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: slang for sexual intercourse

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