soldier n.

c. 1300, souder, soudiour, "fighting man, one engaged ion military service," from Old French soudier, soldier and Anglo-French variants, "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, properly "coin of thick or solid metal," not of thin plate (see solid (adj.)).

The -l- has been regular in English since mid-14c., in imitation of Latin. But the old pronunciation persisted in 16c.-17c. spelling variants sojar, soger, sojour; colloquial sojer appears in print in U.S. Civil War (Willie and Joe always say sojer in the Bill Mauldin World War II cartoons).

Modern French soldat is borrowed from Italian and displaced the older French word; one of many military (and other) terms French picked up during the Italian Wars in early 16c.; such as alert, arsenal, colonel, infantrie, sentinel.

As "one who obeys the commands of and contends in the cause of another," mid-14c., originally especially in the language of faith. Figurative uses 18c.-19c. tended toward notions of "armored," "combative," and, of things and animals, "reddish," for the uniform color. Of ants or termites that take on combat roles, by 1781.

Old slang names for military men circa early 19c. include mud-crusher "infantryman," cat-shooter "volunteer," fly-slicer "cavalryman," jolly gravel-grinder "marine." Soldier boy is attested from 1861. Old soldier "one practiced or experienced" in anything is by 1722.

1640s, "to serve as a soldier," from soldier (n.). The 19c. senses of "malinger, work poorly or hurriedly" seem to be nautical in origin. Related: Soldiered; soldiering. To soldier on "persist doggedly" is attested from 1954.