behind vs rear end

behind and rear end both are nouns.

Word	Noun	Adjective	Verb	Adverb
behind	Yes	Yes	No	Yes
rear end	Yes	No	No	No

As nouns, behind and rear end are synonyms defined as:

behind (noun)	rear end (noun)
the fleshy part of the human body that you sit on	the fleshy part of the human body that you sit on

behind (adjective)	rear end (adjective)
having the lower score or lagging position in a contest

behind (adverb)	rear end (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