In Russian there are many set expressions which initial meaning is forgotten long ago almost by all. At the same time such phrases are still actively used, but already in the acquired allegoric sense. Expression "on the sly" also belongs to them, in particular.
What is "sapa"?
Most of people agree on what to do something "on the sly" - means "imperceptibly, quietly, is reserved, without drawing attention". However the etymology of this expression as it is considered that it has the French origin is interesting. In the Middle Ages of one of the main tasks in the war was to get into the besieged opponent's fortress. Stenobitny cars, assault ladders and also underminings were for this purpose used.
In the French language word sape means a shovel. Began to call also all works connected with the earth: trenches, trenches and underminings. By the way, from this word there was a modern word "sapper" meaning actually not the specialist in explosive business, but the soldier or the engineer of position troops. They in this case are understood as the military connections providing preparation of positions for defense, firing, placement.
The French word sape appeared from Italian where zappa is meant too by "shovel/mattock".
"Silent sapa" and other its types
But why "sapa" – silent? The fact is that undermining under its walls was one of the safest ways to weaken defense of enemy fortress. There were two options of conducting works: opened ("a flying sapa") when the trench was dug under protection of an embankment or barricade, and closed ("a cross over, silent sapa"). In the second case the tunnel was dug directly from positions of the besieging troops without an exit to a surface. The reserved option was more preferable because, having noticed the conducted works besieged could begin to dig a counter tunnel to bring down the undermining going to them. Therefore, sappers had to behave is most reserved. From such way of a ground support action there was also an expression "on the sly" meaning "secretly without drawing attention".
Some experts believe that expression "on the sly" came from the Sanskrit word "sarp" - a snake.
After sappers got under the base of fortifications, they could or provide a reserved exit outside, that is allow besieging secretly to get into fortress, or as a final chord it is simple to roll a part of a tunnel, and together with it and a fortification over it. The second option was more preferable as along a narrow stuffy corridor rather difficult quickly to spend the number of soldiers, necessary for the attack, and for the collapse of a tunnel was to set fire to wooden props enough. After the invention of gunpowder under the base began to plant bombs to strengthen destructions.