"ten-dollar bill," Amer.Eng. slang, 1850, from resemblance of X (Roman numeral 10) to the ends of a sawhorse. Sawbuck in the sense of "sawhorse" only is attested from 1862 but presumably is older (see saw (n.2)).
n. a ten-dollar bill. (From the time when the 10 was indicated by the Roman numeral X (10), which looks like the crosspiece that supports wood that is being sawed.) : It cost me a sawbuck to have my car pulled out of the mud.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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