soldier (n.) Look up soldier at Dictionary.com
c.1300, souder, from Old French soudier, soldier "one who serves in the army for pay," from Medieval Latin soldarius "a soldier" (source also of Spanish soldado, Italian soldato), 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. Modern French soldat is borrowed from Italian and displaced the older French word; one of many military (and other) terms picked up during the Italian Wars in early 16c.; such as alert, arsenal, colonel, infantrie, sentinel.
soldier (v.) Look up soldier at Dictionary.com
"to serve as a soldier," 1640s, from soldier (n.). Related: Soldiered; soldiering. To soldier on "persist doggedly" is attested from 1954.