See quotations.
1841. Flat. Without interest, in brokers’ slang.—N.Y. Standard, Jan.
1870. Stock can almost always be obtained by borrowers, either flat, i. e. with no interest on either side, or with interest at market rates for the money advanced to the lender.—James K. Medbery, ‘Men and Mysteries of Wall Street,’ p. 61 (Boston).
