"in arrears" is derived from "en arrière" which is still used in modern French. Like most French words, it evolved from Latin: ad retro.
In modern French, it's used both literally and figuratively. A common folk song goes "3 pas en arrière…" (3 steps backward). Similarly, "arriérés" could mean "people with retrograde mindsets", or "arrears", i.e. rent or taxes that should have already been paid. I suppose the English language merged "arriérés" and "arrière" into a single word.
In modern French, it's used both literally and figuratively. A common folk song goes "3 pas en arrière…" (3 steps backward). Similarly, "arriérés" could mean "people with retrograde mindsets", or "arrears", i.e. rent or taxes that should have already been paid. I suppose the English language merged "arriérés" and "arrière" into a single word.