arrears (n.)

"balance due, that which is behind in payment," early 15c., plural noun from Middle English arrere (adv.) "in or to the rear; in the past; at a disadvantage" (c. 1300), from Anglo-French arrere, Old French ariere "behind, backward" (12c., Modern French arrière), from Vulgar Latin *ad retro, from Latin ad "to" (see ad-) + retro "behind" (see retro-).

It generally implies that part of the money already has been paid. Arrearage (early 14c.) was the earlier noun. Phrase in arrears first recorded 1610s, but in arrearages is from late 14c.