In the Middle Ages, lots of people lived in wooden houses. Quite understandably, they were very concerned about fires breaking out. Thus, at a specified time every night, a bell would be rung in those medieval villages, as a call that it was time to put out the fires and go to sleep. These were known as curfews. The word is a shortening of the Old French word cuevrefeu, which meant "cover-fire", something you would do to put it out (and no, not to support your military unit). In imperative form, this is literally a portmanteau of "cover" (covrir) and "fire" (feu). Covrir is from Latin cooperere, with the same meaning. Then we eliminate the prefix con- to get operere, which still meant the action of covering something (which made the con- sort of redundant, to be honest). This is from Proto-Indo-European hepi, meaning "near", because PIE is weird. Going back to feu, "fire", is a much shorter journey to us, being a shortening of the Latin word focus, meaning "fireplace", for which we have no etymology. It could be everything from Greek to Armenian to PIE again, so here we give up and accept that the real reason firefighters exist is to impose curfews.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.