A loophole is one of those legally ambiguous things that celebrities (I’m looking at you Take That/Jimmy Carr) exploit to avoid paying tax. The word itself has an interesting backstory, and actually doesn’t have anything to do with loops. OOOH.
Allow me to take you back to the 16th century. There’s peasants and mud everywhere. It’s probably raining. You’re looking up at a medieval stone castle, which has slits in it for people to shoot arrows out of, with little risk of being hit by their attackers (unless they’re Kevin Costner). And these were known as… wait for it… loopholes. But they’re not called this because they (sort of) look like loops. The name comes from the Dutch word lûpen, which means ‘to watch’. So it’s literally a hole to watch out of (I probably didn’t need to explain that, did I?).
It’s not entirely clear how the meaning of loophole changed from window you shoot stuff out of to tax dodging. It’s more likely that the modern sense of loophole is related to actual loops, rather than windows (especially as you close a loophole). There’s also a second theory that it comes from another Dutch word loopgat (which isn’t used anymore), which describes a hole which someone or something could escape through.