n.pl. an amount still outstanding or uncompleted, esp. work undone or a debt unpaid.
Phrases and idioms:
in arrears (or arrear) behindhand, esp. in payment.
Derivatives:
arrearage n.
Etymology: ME (orig. as adv.) f. OF arere f. med.L adretro (as AD-, retro backwards): first used in phr. in arrear