Apparently (according to the Oxford English Dictionary, the term comes from the French "couvre, imper. of couvrir to cover + feu fire." It was originally "A regulation in force in mediæval Europe by which at a fixed hour in the evening, indicated by the ringing of a bell, fires were to be covered over or extinguished." "The primary purpose of the curfew appears to have been the prevention of conflagrations arising from domestic fires left unextinguished at night. The earliest English quotations make no reference to the original sense of the word; the curfew being already in 13th c. merely a name for the ringing of the evening bell, and the time so marked." I did not know that!