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.
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?
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.