Who Are Sappers?
Military engineering is the oldest of the engineering skills and a precursor to the profession of civil engineering.
Employed since ancient and medieval times, military engineers gradually became specialists who made and used engines of war such as catapults, battering rams, and the many other devices used in attacking or defending a castle or camp.
In peace time military engineers designed and built roads, aqueducts, and the fortifications that protected towns and cities.
Military engineers became popularly known as 'sappers' in the 17th century, when attackers dug covered trenches to approach (and then undermine) the walls of a besieged fort. The French word sappe means spadework or trench, hence, those specialist soldiers who dug those trenches became known as ‘Sappers’.
The descriptor ‘Sapper’ is now used as a general term to refer to all military engineers. It is also the equivalent rank title to ‘Private’ for soldiers in the Corps of Royal Australian Engineers.
Source: Encyclopaedia Britannica.