In the 1941 movie classic The Maltese Falcon, Sam Spade is negotiating with Gutman about a remuneration of $25,000 for his services. Later on, Gutman hands him an envelope with $10,000. Spade complains that they were talking about more than this sum.
Gutman replies: “Yes, sir, we were, but this is genuine coin of the realm. With a dollar of this you can buy ten dollars of talk.”
Where does this phrase “genuine coin of the realm” referring to hard cash come from?
I beleive it may be from when Spain first took over the richs of the Aztec and Inca empires and started making coins (reals) to ship back to Spain in order to get an accurate count of thier haul. Hard to tally up jewelry, so they melted it down and made coins.
Problem was, those guys that they sent over here were greedy crooks and started making them a little light. The King of Spain sent some guys over to take care of it by cutting off an few assayers heads and restamping the light coins.
Surely non-genuine coins go back to the first coins, which went out of style with every king.
Each king realized he could be rich by minting money, and thus each coin was quickly devalued.
To the point of the OP, the phrase goes back to at least Shakespeare, according to the OED.
Whether it goes back farther is harder to determine.
In olden times, a variety of money might be circulating–foreign coins, coins issued by private mints, bank notes. But the most widely accepted money, most liable to trade without any discount, was likely to be the current coinage of the royal mint–the genuine “coin of the realm”.
Just as today, all sorts of things have value–stocks, bonds, checks, money orders, credit cards. But nothing is quite like those green pieces of paper printed by the government.
Or in your case, those mutli-colored pieces of paper printed by the European Union.