buttocks vs body part

buttocks and body part both are nouns.

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

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

buttocks (noun)	body part (noun)
the fleshy part of the human body that you sit on	any part of an organism such as an organ or extremity