the.com/sanding block

means A rigid pad (wood, rubber, or plastic) that sandpaper wraps around so pressure spreads evenly across a surface instead of digging in wherever your fingers happen to be.

from No mystery here — 'sand' goes back to Old English 'sand' for the gritty stuff itself, used for scouring long before sandpaper existed, and 'block' comes from Old French 'bloc' (a solid chunk of wood), traceable further to Germanic roots for a tree stump or log. Sandpaper as a manufactured product emerged in the 1800s, and the block followed naturally — someone got tired of the paper puckering and folding under bare fingers and realized a flat chunk of wood solved the problem instantly. The word is simply the sum of its parts: sand's abrasive job, block's steady shape.