In the 14th century, many fortifications were constructed out of stones, especially castles and walls.
The weight of the stones meant that it was hard to provide windows in such structures. Iron and steel were not used in construction and there was no way to support stones in any other way but with other stones. As a result, it was normal to leave a small slit or a gap in the wall to double up as a window. These gaps used to be called loupe.
In the 16th century, these slits in castle walls were used by archers to fire at oncoming enemies without allowing them an opportunity to attack back. These gaps came to be known as loopholes.
While loopholes did give undue advantage to the soldiers within the castle, they had nothing to do with the law. It is unknown how this word came to be associated with the law as it is nowadays.
One of the assumptions is that it is derived from Dutch where the verb loopen means to run.
There is a piece of transformation of this word that nobody seems to have chronicled.
Why link that word to Loupe. Maybe it has some differt origin? What do lexicographers say about it?