I'll have to ask Sam about this one.
I came across this description on Military.com on the origins of the name "Sapper"
"Sapper" got its designation from the 800-1500's engineers who constructed siege engines and dug tunnels on site using standing trees as their primary construction material... hence the oozing sap covered them regularly giving them a recognizable visual indicator of their specialty.
Here is another discription.
Dawn of the Army's Sappers
A SAPPER is "a soldier employed in working at saps, the building and repairing of fortifications, the execution of field-works, and the like." A 'SAP' is "the process of undermining a wall or defensive work,", "the process of constructing covered trenches in order to approach a besieged place without danger from the enemy's fire,", or "a covered trench made for the purpose of approaching a besieged place under the fire of the garrison."
Generals in the 16th century learned that trenches and tunnels dug towards the walls of an enemy's fortification could provide protection from defensive fire. Either large guns or explosive charges could then be employed at close range to breach the defenses and cause a climactic and decisive end to the battle. Technological advances allowed the development of these heavy bronze and cast-iron artillery pieces which the generals moved into postion under cover of the trenches. The trenches were dug with a small hook shaped spade which was called in French a "sapier" and when these trenches were used for the attackers advance, they were called sap trenches, or "saps". The men who dug these "saps" were affectionately called SAPPERS; and a legacy was born. The SAPPERS who were called upon to place explosive charges under the enemy's fortifications were called MINERS.
Desiring soldiers in the Continental Army who were skilled in such fashion, The Corps of Sappers and Miners were created by a General Order of none other than General George Washington on the Second of August, 1779. The SAPPERS and MINERS, having already been formed into companies the previous Summer were already receiving their introduction to the Sapper Guild. They were an Elite group and as they are today, they were placed under the direction of the Corps of Engineers. Their purpose for existance was to prepare defenses and attack and break through enemy defenses.
The SAPPERS of the Continental Army were the first troops into the British lines at the decisive battle of the American Revolution;The Battle of Yorktown in October 1781. At Yorktown, the SAPPERS assembled fortification materials, erected gun platforms, transported cannons and ammunition, and cleared the way in their customary manner for their Infantry brothers in the classic assault on Redoubt Number 10. This victory would not have been brought home had it not been for the SAPPERS; and there would have been no United States of America.