whetstone; knife sharpener
A stone used for sharpening blades, especially kitchen knives. 砥石 (といし) is a common synonym.
I bought a new whetstone to sharpen my kitchen knives.
Compound of 研ぎ (togi, 'sharpening', the continuative form of 研ぐ togu) + 石 (ishi, 'stone').