The Collaborative International Dictionary of English v.0.48:

Sinapism \Sin"a*pism\, n. [L. sinapismus, Gr. ????, the use of a
   mustard blister, fr. ??? to apply a mustard blister, fr. ????
   mustard.] (Med.)
   A plaster or poultice composed principally of powdered
   mustard seed, or containing the volatile oil of mustard seed.
   It is a powerful irritant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sinapism
    n 1: a plaster containing powdered black mustard; applied to the
         skin as a counterirritant or rubefacient [syn: mustard
         plaster, sinapism]