Originally from the Californian gold mines, digger made its way to our Australian goldfields with miners from the US of A in the 1850’s.
At its core digger represented mateship. In contrast to the more solitary prospector, the term digger recognised the hardships of mining could be faced more successfully by working together.
During World War 1 digger became associated with the trench digging Australian and New Zealand infantry on the frontline. The term for British privates in the same role was sapper, from the French saper (to undermine, to dig under a wall or building to cause its collapse).
Although digger obviously referred to the endless task of establishing trenches, it also carried the strong sense of mateship shared by our soldiers. In this sense the use spread to include all Australian and New Zealand soldiers and was adopted as a form of greeting.
Lest we forget.