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, 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. Willie and Joe always say sojer in the Bill Mauldin World War II 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.

Old slang names for military men circa early 19c. include mud-crusher "infantryman," cat-shooter "volunteer," fly-slicer "cavalryman," jolly gravel-grinder "marine."

"infantryman, soldier who serves on foot," 1620s, from foot (n.) + soldier (n.).

1560s, from French souderie or else a native formation from soldier + -y (1).

also solə-, Proto-Indo-European root meaning "whole, well-kept." 

It forms all or part of: catholic; consolidate; consolidation; holism; holo-; holocaust; Holocene; hologram; holograph; insouciant; safe; safety; sage (n.1) kind of herb; salubrious; salutary; salute; salvage; salvific; salvo "simultaneous discharge of guns;" save (v.) "deliver from danger;" save (prep.) "except;" solder; soldier; solemn; solicit; solicitous; solid; solidarity; solidity; sou.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sarvah "uninjured, intact, whole;" Avestan haurva- "uninjured, intact;" Old Persian haruva-; Greek holos "whole;" Latin salvus "uninjured, in good health, safe," salus "good health," solidus "solid;" Armenian olj "whole, healthy."  

"native of India in British military service," 1717, from Portuguese sipae, "native soldier," in distinction from a European soldier, from Urdu sipahi, from Persian sipahi "soldier, horseman," from sipah "army." The Sepoy Mutiny against British rule in India took place 1857-8.

1620s, of persons, "to serve as a soldier" (now rare), from Latin militatum, past participle of militare "serve as a soldier," from miles "soldier" (see military (adj.)). The sense developed via "to be in conflict with, be at variance" to "be evidence" for or against, "have weight or force in determining anything" (1640s). Related: Militated; militating; militation.