"ten-dollar bill or note," also saw-buck, American English slang, 1850 (implied in double-sawbuck), so called from the resemblance of X (the Roman numeral 10, prominent in the design of many mid-19c. U.S. bank notes) to the ends of a sawhorse. Sawbuck in the sense of "sawhorse" is attested by 1837 (see saw (n.1) + buck (n.3)).
At the foot of the flag staff four saplings were erected, somewhat after the form of the two ends of a "saw-buck," and not very unlike the characters that denote the value of a ten dollar bill .... [Maumee Express, Maumee City, Ohio, Oct. 21, 1837]
updated on January 17, 2022