upper millstone
Refers to the top stone in a traditional mill or grinding apparatus. A specific, concrete noun.
石臼の上石を回して大豆を挽く。
Turn the upper millstone of the stone mortar to grind soybeans.
Compound of 上 (うわ, upper) and 石 (いし, stone). The reading うわ is a combining form of 上 (うえ).