[ahartmetz]

It comes from Latin fenestra. People are throwing around lots of languages in this subthread, but it needs to be said that the circonflex indicates an omitted s from the Latin original.

Maybe there are cases where the original isn't Latin, but I've never noticed one, and French does not have many words of non-Latin origin. I'm not sure if English speakers commonly know that, but English is a rare case of thoroughly mixed origins.