The Collaborative International Dictionary of English v.0.48:

Menstruum \Men"stru*um\, n.; pl. E. Menstruums, L. Menstrua.
   [L. menstruus. See Menstruous.]
   Any substance which dissolves a solid body; a solvent.
   [1913 Webster]

         The proper menstruum to dissolve metal.  --Bacon.
   [1913 Webster]

         All liquors are called menstruums which are used as
         dissolvents, or to extract the virtues of ingredients
         by infusion or decoction.                --Quincy.
   [1913 Webster]

   Note: The use is supposed to have originated in some notion
         of the old chemists about the influence of the moon in
         the preparation of dissolvents. --Johnson.
         [1913 Webster]