Lighter, Historical Dictionary of American Slang- Earliest quote for 'frogskin is 1902. "Dialect Notes 2"- "Frogskins is used in Virginia for paper money or 'greenbacks'". Usually applied now to the one-dollar bill, but also used in the plural for money. 'Frogskins' was used by Hollywood in their horse operas with trapper and Indian characters and entered general speech.

"Greenback" was applied to Demand notes ($1 value) which were first produced in 1861, and appeared in the Century Dictionary in the 1880's. It possibly was used during the Civil War but no definite citations found yet. U. S. currency is green on the reverse side and black and gray on the obverse.