1550, "reasoning," later, "relation of one number to another" (1660s), then "fixed allowance of food" (1702, often rations, from French ration in this sense), from Latin rationem (nominative ratio) "a reckoning, calculation, proportion" (see ratio). The military pronunciation (rhymes with fashion) took over from the preferred civilian pronunciation (rhymes with nation) during World War I.

"put (someone) on a fixed allowance," 1859, from ration, n.; sense of "apportion in fixed amounts" is from 1870. Related: Rationed; rationing.