upper millstone
Refers to the top stone in a traditional stone mill, used for grinding grains. A specialized term encountered in historical or agricultural contexts.
Turn the upper millstone of the stone mill to grind soybeans into powder.
Compound of 上 (うわ, 'upper') and 臼 (うす, 'mortar' or 'millstone'). The reading うわ is a combining form of うえ.