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.