栓 means 'plug'

Bathing in Japan could mean communing with nature, following rigid procedures in a bathhouse, or soaking in water that other relatives have used. These essays cover all situations. They introduce practical terms for "bathtub," "turning on a faucet," and "overflowing." You'll learn where people bathed before owning bathtubs. You'll see how, in bathhouses, smaller, raised pools and larger pools below floor level respectively represent land and sea. You'll learn which "bathtub" term the Japanese associate with empty basins versus full pools at hot springs. And you'll discover how bathing has connected to religion, family bonds, prostitution, and a famous thief.