means The act of smoothing or abrading a surface by rubbing it with sandpaper or another gritty abrasive.

from From "sand" — the gritty stuff of beaches and deserts — which traces back to Old English "sand," with cousins across the Germanic languages (Dutch "zand," German "Sand"). The verb grew naturally from the noun: long before manufactured sandpaper, craftsmen literally rubbed surfaces smooth with sand, sometimes glued to leather or cloth. The "-ing" simply turns the act into the ongoing process — the dust-making itself.