saw ‧ buck /ˈsɔːbʌk $ ˈsɒː-/ BrE AmE noun [countable] American English old-fashioned informal
[ Date: 1800-1900 ; Origin: sawbuck 'sawhorse' (19-20 centuries) , from Dutch zaagbok ; probably because the end of some types of sawhorse looks like a Roman number X (= 10) ]
a piece of paper money worth $10