curfew

From Middle English curfu, from Old French cuevre-fu, from the imperative of covrir ("to cover") plus fu ("fire").

The word began as a literal instruction to cover or extinguish fire, and came into English carrying that same command before settling into its modern sense of a set time to be off the streets or indoors.

It's related to kerchief, which shares this same French ancestry through the idea of covering.