1727, "action of assorting according to quality," from French triage "a picking out, sorting" (14c.), from Old French trier "to pick, cull" (see try (v.)). There seems to be some influence from or convergence with Latin tria "three" (as in triage for "coffee beans of the third or lowest quality"). In World War I, adopted for the sorting of wounded soldiers into groups according to the severity of their injuries, from French use.
First of all, the wounded man, or "blessé, is carried into the first of the so-called "Salles de Triage" or sorting wards. Here his name and regimental number, and if he is in condition to give it, the address of his family are taken; .... Then a hasty look-over from the surgeon sends him into one of the two other "Salles de Triage" -- that of the "Petits Blessés" if he is only slightly wounded and that of the "Grands Blessés" if he is more severely so. [Woods Hutchinson, M.D., "The Doctor in War," Boston, 1918]