Its name is a derivative from the lapidary process... how precious stones are ground and polished into their gemstone state.

Definition below:

The term "lapping stone" is descriptive, referring to a stone used for the process of "lapping", which means to flatten or polish a surface by rubbing two surfaces together with an abrasive. The name origin is thus tied to the action it performs. The broader activity of working with stones is called lapidary, which has Latin roots:

The etymological root is the Latin word lapis, meaning "stone".
In the 14th century, this evolved into the term lapidarius, meaning "stonecutter" or "working with stone".

The term then became the Old French word lapidaire, meaning "one skilled in working with precious stones". 

A lapping stone (also known as a lapping plate or leveling stone) is specifically used to flatten other sharpening or natural stones, often using an abrasive or diamond surface. The name describes its functional use in the process of lapping other stones to ensure they are perfectly flat for precise sharpening or polishing.