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.
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         For much I dread due payment by the Greeks
         Of yesterday's arrear.                   --Cowper.
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         I have a large arrear of letters to write. --J. D.
                                                  Forbes.
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   In arrear or In arrears, behind; backward; behindhand; in
      debt.
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