Merriam-Webster Collegiate Dictionary - arrear

 noun  Etymology: Middle English arrere behind, backward, from Anglo-French arere, from Vulgar Latin *ad retro backward, from Latin ad to + retro backward, behind — more at at, retro-  Date: 1620  1. the state of being behind in the discharge of obligations — usually used in plural in ~s with the rent  2.  a. an unfinished duty — usually used in plural ~s of work that have piled up  b. an unpaid and overdue debt — usually used in plural paying off the ~s of the past several months