ass vs piece of ass

ass and piece of ass both are nouns.

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

As nouns, ass and piece of ass are synonyms defined as:

Other synonyms of ass include fuck, fucking, nookie, nooky, piece of tail, roll in the hay, screw, screwing, shag, shtup.

ass (noun)	piece of ass (noun)
slang for sexual intercourse	slang for sexual intercourse
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