Loophole has several definitions:
—an arrow slit in a castle wall, a narrow vertical window to shoot from
—a technicality that allows escape from a contract or commitment
—a method of escape, especially an ambiguity or exception in a rule
—an ambiguity or inadequacy in a system, which can be used to circumvent it
In the 1300s, an English castle would have had several “loupes,” 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 only to arrow-slits.
The earliest published reference in the Oxford English Dictionary is from William Langland’s narrative poem Piers Plowman, written in the second half of the 1300s.
The word “loupe” may have come from a Middle Dutch word, lupen, meaning to lie in wait, or to watch. The word eventually evolved from “loupe” to “loophole.”
In the 1600s, “loophole” began to be used figuratively for a “means of escape” and by 1700 for an ambiguity or inadequacy that allowed somebody to evade the provisions of laws and rules.
Today, loopholes are searched for and used strategically in a variety of circumstances, including elections, politics, taxes, the criminal justice system, or in breaches of security.
Apparently the military is still instructed in “loopholes” but these are usually existing windows or holes deliberately made to serve as firing ports when fighting in an urban environment.
To knit a sweater, one connects thousands of little loops of wool. But I suppose they are only loopholes while they’re on the needle.