Gold coins. This is also a piece of English slang.

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1841.  This was the currency, or what is its equivalent, and what the modern Whigs deride, and sneeringly call the “Tom Benton humbug,” I mean the yellow boys.—Mr. Duncan of Ohio, House of Repr., Jan. 26: Cong. Globe, p. 157, App.

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1841.  Mr. Henry Clay alluded to the “Jackson notions of yellow-boy currency.”—U.S. Senate, July 7: id., p. 129.

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1861.  

        Now I hain’t seen one in six munts, for, sence our Treashry Loan,
Though yaller boys [mulattoes] is thick enough, eagles hez kind o’ flown.
Lowell, ‘Biglow Papers,’ 2nd Series, No. 1.    

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