nether millstone; bottom millstone
Refers to the stationary lower stone in a traditional mill. A somewhat specialized term; in everyday contexts, 石臼の下石 or 挽き臼の下石 may be used for clarity.
In the watermill, the nether millstone firmly supported the rotating upper millstone.
Compound of 下 (shita, 'below') and 石 (ishi, 'stone'), with rendaku voicing of the second element. The reading したいし is a straightforward combination of the kun'yomi readings.