Emily Dickinson Lexicon

AR-REAR', adv. [Fr. arriere, behind. In some of its uses it has the sense of lower, inferior. (See Arriere-ban.) Sp. and Port. arriar, to lower sail; Arm. reor, revr, or refr, the fundament; W. rhevyr, id., from rhev, thick. Lunier deduces arrear and arriere from L. ad and retro. But the derivation from the Celtic seems most probably correct.]

Behind; at the hinder part. – Spenser. In this sense obsolete. But from this use, we retain the word as a noun in the phrase, in arrear, to signify behind in payment.