I found an old diary from the 1800s where the writer describes how he almost died but was saved by a sinapism. What is that?

A sinapism is a mustard plaster (or poultice) — an old-fashioned remedy that speeds up healing. The mustard plaster is a paste made from powdered mustard and other ingredients, spread on a cloth, and applied to the skin. It always has to be wrapped in a cloth; if you apply the mustard paste directly to the skin, it can cause burns or blistering.

In Madame Bovary, Emma Bovary becomes seriously ill after her lover breaks off the illicit relationship. Her husband Charles, a country doctor, attends to her with devotion:

For forty-three days Charles did not leave her. He gave up all his patients; he no longer went to bed; he was constantly feeling her pulse, putting on sinapisms and cold-water compresses.