ass vs body part

ass and body part both are nouns.

Word	Noun	Adjective	Verb	Adverb
ass	Yes	No	No	No
body part	Yes	No	No	No

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

ass (noun)	body part (noun)
slang for sexual intercourse	any part of an organism such as an organ or extremity
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