whetstone, grindstone
砥 centers on a stone used for sharpening or grinding: a whetstone or grindstone. Both meanings refer to the same abrasive tool.
砥 combines 石 (stone) with 氐, which likely contributes the sound and relates to a base or foundation, suggesting a stone used as a base for sharpening.
The left side is a stone 石, and the right side 氐 looks like a base or foundation. Picture a flat stone base used to sharpen blades: a whetstone.
For シ, imagine sharpening a blade on a whetstone and hearing the soft 'shhh' sound of metal against stone: shhh -> シ.
whetstone; grindstone
whetstone; grindstone
medium-grade millstone
medium-grade millstone
grindstone
razor strop; hone
double whetstone
oilstone
sludge from sharpening a sword, etc. on a grindstone
rough grindstone
Aoto Zōshi Hana no Nishiki-e (kabuki play by Kawatake Mokuami)
leather strop