grindstone sludge; sword-sharpening residue
Refers to the muddy, abrasive slurry produced when sharpening a blade on a whetstone. A technical or traditional craft term, not used in everyday conversation.
After sharpening the sword, wipe off the grindstone sludge thoroughly.
Compound of 砥 (grindstone, whetstone) and 糞 (excrement, waste), literally 'grindstone waste'. The exact historical derivation is uncertain, but the term is a straightforward description of the residue.