Q From Will Thomas: Where do we get loophole from?
A A typical medieval English castle would have had — in addition to barbicans, machicolations, crenellations, a portcullis or two and other useful features — a number of loops.
This loop isn’t a “doubling or return into itself of a portion of a string, cord, thong, or the like, so as to leave an aperture between the parts”, as the Oxford English Dictionary explains it. (Defining geometric shapes is a good test of a lexicographer’s skill. It may remind you of the trouble Dr Johnson had with network: “Any thing reticulated or decussated, at equal distances, with interstices between the intersections.”)
These castle-type loops were small gaps or holes in the fortified walls for keeping watch, for archers to shoot through, or to let light into a chamber. Later, the word was applied to arrow-slits to the exclusion of the other senses.
There’s no connection between the two meanings of loop, though one nineteenth-century scholar did attempt to square the semantic circle by suggesting that the apertures were in the shape of loops. It’s likely, the experts suggest, that it comes from the old Dutch verb lûpen, to watch or peer, or glupen, to spy or lurk, to watch with narrowed eyes, whose source is a word for a crack or slit.
In the sixteenth century, loop began to be expanded to loophole. It seems that Englishmen were as puzzled and confused then by the two senses of loop as we might be today and added the second part to make it clear they were talking about openings in walls and not doubled-over bits of string.
Around the middle of the following century loophole began to be used figuratively for a means of escape and by 1700 could have our modern sense of an ambiguity or inadequacy in rules or laws that allows somebody to evade their provisions.