lower millstone; nether millstone
The stationary bottom stone in a traditional mill. Often contrasted with 上臼 (うわうす, upper millstone). The cross-reference 下石 (したいし) is a synonym.
See also: 下石
In the watermill, the lower millstone is fixed in place without moving.
Compound of 下 (した, lower) and 臼 (うす, millstone). The reading is a straightforward kun'yomi combination.