Dr. Dominique Jean Larrey (1766-1842) first used triage in a medical sense as surgeon-in-chief of Napoleon's armies from 1797 to the battle of Waterloo in 1815.
The great doctor sorted battlefield casualties into three groups: 1) patients who would probably die, no matter what; 2) patients who would most likely live, no matter what; 3) patients who would survive with immediate treatment, but would not survive without.
And, using the model of artillery carriages, drawn by two- or four-horse teams, that "flew" over the battlefield into position, Dr. Larrey came up with flying ambulances, ambulances volantes. The crew of the flying ambulances administered first aid to the wounded and then rushed them to nearby surgical waystations, the forerunners of MASH units, for emergency treatment.
In World War I, English and American medical personnel heard the French using triage for this effective method of saving badly wounded soldiers and so, thanks to the wonderful flexibility of the English language, it became our word as well.
Wounded men by the side of the road, Battle of Passchendaele.
Frank Hurley - State Library of New South Wales file:a479035.