Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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