also: かいしでい
noun
mustard plaster
A traditional poultice made from mustard powder; used as a folk remedy. Rare term.
昔は風邪をひくと芥子泥を胸に貼ったものだ。
In the old days, people would apply a mustard plaster to the chest when they caught a cold.
Compound of 芥子 (mustard) and 泥 (mud, paste).