Mustard plaster
A mild mixture of mustard seed powder, an adhesive such as flour, and water warmed and spread on a moist cloth that is then applied to the skin.
The original mistaken notion was that the mustard plaster would draw out the 'bad humors.' It acts as a counterirritant and increases the local blood supply.
A mustard plaster is a form of poultice.
See also: Cataplasm