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.
Cite This Source
Word Origin & History
sawbuck
"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.
[saw-buhk] 
