Answer: Castle Architecture 

In modern use the term “loophole” is used to refer to an ambiguity, shortcoming, or exploit available in legal or security systems. One often hears of tax “loopholes” that allow large corporations to avoid paying taxes in their country of origin, for example, or a loophole that allows a criminal to avoid trial.

Historically, loopholes had nothing to do with legal code or the like, but everything to do with the design of castle architecture. Loopholes, or arrow slits, were narrow windows carved out of castle walls that allowed defending archers to shoot castle attackers while remaining almost entirely protected.

Later, the term was adapted to mean using the existing system to your advantage (much like the archers used the design of the castle walls to their advantage).