Clues we might use for this word include ‘Night-time outings restriction’ or ‘After-hours travel ban’, but a curfew started as a law aimed at preventing villages burning down.
The word’s origin is in the Old French couvre-feu meaning ‘cover-fire’. In medieval times fires were precious for lighting, heating and cooking. Crude wooden houses with thatched roofs and fires burning in the hearth meant disaster was never far away. Every evening at around 8 or 9 a bell would sound as a signal that it was time to cover fires and settle in for the night, thus assuring no one went to sleep leaving their fire unguarded.
Curfew quickly came to refer to the signal itself . The benefit of restricting the population to their houses was soon seen to extend beyond avoiding destructive fire, to minimising crime and stopping schemers meeting under cover of darkness.
Curfews are now mostly put in place in times of unrest, though in some places they are imposed in order to deter juvenile delinquency.