arse vs body part

arse and body part both are nouns.

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

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

arse (noun)	body part (noun)
vulgar slang for anus	any part of an organism such as an organ or extremity
the fleshy part of the human body that you sit on