A paper banknote, especially a U.S. dollar bill, named for the green ink printed on the back of the bill.
From green + back, referring to the distinctive green coloring on the reverse side of U.S. currency, which began in the 1860s. The term emerged during the Civil War when the U.S. government first issued paper money with green ink on the back to prevent counterfeiting.
The U.S. government chose green ink because it was believed to be the hardest color to counterfeit at the time—but also because green dye was cheap and readily available! This simple practical choice created a term that's been slang for American money for over 160 years.
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