Paper money, especially U.S. dollar bills; cash.
From the green ink printed on the back of U.S. paper currency starting in the 1860s. The term 'greenback' became slang for American money because of this distinctive feature.
The U.S. printed green ink on the back of paper money for a practical reason—it was cheap, durable, and hard to counterfeit—but the side effect was accidentally creating one of the best slang terms for money in any language!
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