In the Bible (Judges 7:5): any of three hundred soldiers selected by Gideon to fight the Midianites. Hence allusively: any particularly devout or zealous soldier.
With reference to the story in which God directs Gideon to order his army to drink, and to select those soldiers who lap the water, as opposed to those who kneel down to drink.
Early 17th century. From water + lapper.