behind vs in arrears

behind is a noun but in arrears is not a noun.

Word	Noun	Adjective	Verb	Adverb
behind	Yes	Yes	No	Yes
in arrears	No	No	No	Yes

As adverbs, behind and in arrears are synonyms defined as:

Other synonyms of behind include behindhand.

behind (noun)	in arrears (noun)
the fleshy part of the human body that you sit on

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

behind (adverb)	in arrears (adverb)
in or to or toward the rear	in debt
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