Sawbuck is American slang for a ten-dollar bill, heard at casino tables as well as on the street. The name comes from the sawbuck, a wooden sawhorse with X-shaped legs at each end that holds timber for cutting. The X matches the Roman numeral for ten, which appeared on some US ten-dollar notes in the mid-nineteenth century, and this is widely believed to explain the nickname. A double sawbuck is a twenty-dollar bill.
Players still use the word when buying in, tipping or calling a bet, so dealers and cage staff in US-facing venues hear it regularly. For online operators, slang like this matters mostly in live dealer chat and customer support, where staff who understand player vocabulary can respond faster and more naturally. See also dime, slang for a much larger sum.