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]

WordNet (r) 3.0 (2006):

menstruum
    n 1: (archaic) a solvent
    2: the monthly discharge of blood from the uterus of nonpregnant
       women from puberty to menopause; "the women were sickly and
       subject to excessive menstruation"; "a woman does not take
       the gout unless her menses be stopped"--Hippocrates; "the
       semen begins to appear in males and to be emitted at the same
       time of life that the catamenia begin to flow in females"--
       Aristotle [syn: menstruation, menses, menstruum,
       catamenia, period, flow]