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]

The Collaborative International Dictionary of English v.0.48:

Sawhorse \Saw"horse`\, n.
   A kind of rack, shaped like a double St. Andrew's cross, on
   which sticks of wood are laid for sawing by hand; -- called
   also buck, and sawbuck.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sawbuck
    n 1: a framework for holding wood that is being sawed [syn:
         sawhorse, horse, sawbuck, buck]