Your sin, i.e. the object and matter of your sin, as sin is taken Isaiah 31:7. I cast the dust thereof into the brook, that there might be no monument or remembrance of it left.
Related Commentaries of Deuteronomy 9:21
Deuteronomy 9:21
21 And I took your sin, the calf which ye had made, and burnt it with fire, and stamped it, and ground it very small, even until it was as small as dust: and I cast the dust thereof into the brook that descended out of the mount.