a black mixture of mustard and rubber placed on a cloth and applied to the skin as a counterirritant.
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British Dictionary definitions for mustard plaster
med a mixture of powdered black mustard seeds and an adhesive agent applied to the skin for its relaxing, stimulating, or counterirritant effects
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012