from
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]