behind vs body part

behind and body part both are nouns.

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

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

behind (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

behind (adjective)	body part (adjective)
having the lower score or lagging position in a contest

behind (adverb)	body part (adverb)
in or to or toward the rear
remaining in a place or condition that has been left or departed from
in debt
in or into an inferior position
showing a time that is earlier than the actual time