Soldiering on

“Soldier” and “solid” both come from the Latin word solidus (whole, complete). A solidus nummus was a “complete coin”, that is, a solid gold coin. Shortened to solidus and then to “sold” (which is related to "solder", but not the past tense of "sell", which is another word entirely), it was borrowed into English in the 1300s to designate money paid to people in the army, who therefore became “soldiers”.