Latin solidus: solid, firm, and the name of a late Roman gold coin. Soldarius became the man paid in solidi, the professional fighter as opposed to the levied farmer. Old French soldat, English soldier: the mercenary origin is not hidden, it is the word itself.
The reading every veteran recognizes: the name admits the transaction. Rome's legions were history's first large standing professional army, and language recorded the innovation honestly, these are the paid ones.
The adjective's other children: solid, consolidate, solder (to make solid). The soldier and the solder joint share a promise, to hold firm, and the coin was called solid because it held its value when others debased. Firmness as the family trade, in metal, metalwork, and men.
Is the army a profession that got named after its payroll, or a payroll that grew legs?