The word curfew originates in the medieval practice of ringing a bell at a fixed time in the evening as an order to bank the hearths and prepare for sleep. It comes from the Anglo-Norman coeverfu, the equivalent of the Old French cuevre-fe, or cover fire.
We can see the Anglo-Norman in the 1285 Statutes of London:
Apres Coeverfu personé a Seint Martyn le graunt.
(After Curfew person on Saint Martin the great.)
And the English a few decades later in the Seuyn Sages, c.1320:
Than was the lawe in Rome toun, That, whether lord or garsoun That after Corfu be founde rominde, Faste men scholden hem nimen and binde.
(Then was the law in Roman towns, that, when a man or boy is found roaming about after curfew, men should take and bind him fast.)
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2016, by David Wilton