a U.S. legal-tender note, printed in green on the back since the Civil War, originally issued against the credit of the country and not against gold or silver on deposit.
n. a dollar bill. : It's only ten greenbacks. Anybody can afford that.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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"U.S. dollar bill," 1862, so called from the time of their introduction; bank paper money printed in green ink had been called this since 1778 (as opposed to redbacks, etc.).