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c.1300, souder, from Old French soudier, soldier "one who serves in the army for pay," from Medieval Latin soldarius "a soldier" (cf. Spanish soldado, Italian soldato and French soldat "soldier," which is borrowed from Italian), literally "one having pay," from Late Latin soldum, extended sense of accusative of Latin solidus, name of a Roman gold coin (see solidus). The -l- has been regular in English since mid-14c., in imitation of Latin. Willie and Joe always say sojer in the Bill Mauldin cartoons, and this seems to mirror 16c.-17c. spellings sojar, soger, sojour.
"to serve as a soldier," 1640s, from soldier (n.). Related: Soldiered; soldiering. To soldier on "persist doggedly" is attested from 1954.