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1.
The watery substance that comes out of the mustard container if you don't shake hard enough before squirting.
Gross, there is premustard all over my buns. Didn't you shake it hard enough?
by Debica July 30, 2010
 
2.
The clear milky discharge that is often released prematurely from the tip of a mustard bottle when squeezed too hard or too quickly.
Bill's bun got splattered with pre-mustard when Lisa tried to apply the French's to his wiener.
by PGM1 March 09, 2010