The Collaborative International Dictionary of English v.0.48:

Arrear \Ar*rear"\, n.
   That which is behind in payment, or which remains unpaid,
   though due; esp. a remainder, or balance which remains due
   when some part has been paid; arrearage; -- commonly used in
   the plural, as, arrears of rent, wages, or taxes. --Locke.
   [1913 Webster]

         For much I dread due payment by the Greeks
         Of yesterday's arrear.                   --Cowper.
   [1913 Webster]

         I have a large arrear of letters to write. --J. D.
                                                  Forbes.
   [1913 Webster]

   In arrear or In arrears, behind; backward; behindhand; in
      debt.
      [1913 Webster]

WordNet (r) 3.0 (2006):

in arrears
    adv 1: in debt; "he fell behind with his mortgage payments"; "a
           month behind in the rent"; "a company that has been run
           behindhand for years"; "in arrears with their utility
           bills" [syn: behind, behindhand, in arrears]