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.
The proper menstruum to dissolve metal. --Bacon.
All liquors are called menstruums which are used as
dissolvents, or to extract the virtues of ingredients
by infusion or decoction. --Quincy.
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.