Emily Dickinson Lexicon

MEN'STRU-UM, n. [plur. Menstruums. From L. mensis, month. The use of this word is supposed to have originated in some notion of the old chimists, about the influence of the moon in the preparation of dissolvents. Johnson.]

A solvent; any fluid or subtilized substance which dissolves a solid body. All liquors are called menstruums which are used as dissolvents, or to extract the virtues of ingredients by infusion or decoction. Quincy. Inquire what is the proper menstruum to dissolve a metal. Bacon.