'Soldier' literally means 'one who fights for a solidus' — named for the Roman gold coin. All soldiers are mercenaries.
A person who serves in an army; one engaged in military service.
From Middle English 'soudier,' from Old French 'soudier' or 'soldier' (one who serves in an army for pay), from 'soude' or 'solde' (pay, wages, especially military pay), from Late Latin 'solidus' (a gold coin). A soldier is literally 'one who fights for a solidus' — a mercenary defined by payment rather than duty, honour, or feudal obligation. The Latin 'solidus' was a standard Roman gold coin introduced by Constantine I around 309 CE, named from 'solidus' (solid
A 'soldier' is literally 'someone paid with a coin.' The Roman solidus that gave the soldier his name also produced the French 'sou' (a small coin), Italian 'soldo' (penny), and even 'solder' — metal made solid. The British abbreviation 's' for shillings (as in '£/s/d') stood for 'solidus.'