(n.) a loophole or embrasure in the wall of a castle or similar building, through which an arrow can be fired
Here’s a great etymological tale: meurtrière is another name for one of those narrow, elongated slits that you see in the walls of castles. Their shape allows them to be used as a lookout point, but also remain almost entirely protected against attacks from outside, and allow for arrows to be shot through them. All told, they’re a fairly clever idea.
They also have a brilliantly evocative name: showing just how effective these shafts could be, the word meurtrière literally means ‘murderess’ in French.
That’s a word that English picked up from French, unsurprisingly—though long after these kinds of defences were ever being used. The Oxford English Dictionary has no record of meurtrière in English before it was included in a handful of nineteenth century guides to ancient castles and and glossaries of military techniques.
In its native French, however, meurtrière dates back to the 1400s, when these murderous loopholes would indeed have been in use.