noun
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.
水車小屋で、下石が回転する上石をしっかりと支えていた。
In the watermill, the nether millstone firmly supported the rotating upper millstone.