The Collaborative International Dictionary of English v.0.48:

Sawbuck \Saw"buck`\, n.
   1. A sawhorse.
      [1913 Webster]

   2. [Colloq., from the Roman X for ten, like the support of a
      sawbuck.] a ten-dollar bill; also,

   double sawbuck, a twenty-dollar bill.
      [PJC]