Is it ensure, insure, or assure?
"ten-dollar bill," American English slang, 1850, from resemblance of X (Roman numeral 10) to the ends of a sawhorse. Sawbuck in the sense of "sawhorse" is attested only from 1862 but presumably is older (see saw (n.1)).
A ten-dollar bill; ten dollarsRelated Terms
[1850+; fr the resemblance of the Roman numeral X to the ends of a sawhorse]