In the medieval towns of Europe, fire was an ever-present danger.
The houses were of timber, usually with thatched roofs, and the household fire, in a hole in the middle of the floor, sent out its smoke and sparks through an opening in the center of the roof.
Naturally, the danger was greatest at night. Therefore, the authorities of each town had a signal sounded, usually a bell, at about the time that folks were about to go to bed, to warn them to take care of their fires for the night.
In many places of Europe this signal was known by the Latin words pyritegium or ignitegium (from pyra or ignis, fire, and tego, cover), but in olden France the signal was called cuevre-feu, cover-fire.
The custom was taken from France to England several centuries before the Norman Conquest, and the people undoubtedly thought they were still speaking its French name when they called it curfew.