A sawbuck is a ten-dollar bill. These bills used to have two large roman numeral tens ("X") on their reverse side, resembling the scaffold for a sawbuck or sawhorse. From The Knickerbocker magazine of 1850:
Send me the two double “saw-bucks.”
Sawbuck is not known to be related to buck, meaning a dollar, but the possibility cannot be dismissed. They certainly influenced one another in any case.
(Source:Oxford English Dictionary, 2nd Edition)
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