Sawbuck is a slang term used both inside and outside casinos to refer to a $10 bill. While the term is almost always used for US dollar bills, it can also be used to refer to other denominations of the same value.
One of the main reasons why the term was coined was due to the design of the sawbuck. The device is used to cut firewood and has an “X” form on both ends. As you may know, X is the Roman numeral that refers to the number 10. Coincidentally, the exact same Roman numeral appeared on the back of a 10-dollar bill from the 1850s. While the exact origins aren’t precise, it is speculated that this is how the term was first coined.
Quite often, you might hear someone using the term double sawbuck, which is used to refer to a twenty-dollar bill.