The Collaborative International Dictionary of English v.0.48:

Mercury \Mer"cu*ry\, n. [L. Mercurius; akin to merx wares.]
   1. (Rom. Myth.) A Latin god of commerce and gain; -- treated
      by the poets as identical with the Greek Hermes, messenger
      of the gods, conductor of souls to the lower world, and
      god of eloquence.
      [1913 Webster]

   2. (Chem.) A metallic element mostly obtained by reduction
      from cinnabar, one of its ores. It is a heavy, opaque,
      glistening liquid (commonly called quicksilver), and is
      used in barometers, thermometers, etc. Specific gravity
      13.6. Symbol Hg (Hydrargyrum). Atomic weight 199.8.
      Mercury has a molecule which consists of only one atom. It
      was named by the alchemists after the god Mercury, and
      designated by his symbol, [mercury].
      [1913 Webster]

   Note: Mercury forms alloys, called amalgams, with many
         metals, and is thus used in applying tin foil to the
         backs of mirrors, and in extracting gold and silver
         from their ores. It is poisonous, and is used in
         medicine in the free state as in blue pill, and in its
         compounds as calomel, corrosive sublimate, etc. It is
         the only metal which is liquid at ordinary
         temperatures, and it solidifies at about -39[deg]
         Centigrade to a soft, malleable, ductile metal.
         [1913 Webster]

   3. (Astron.) One of the planets of the solar system, being
      the one nearest the sun, from which its mean distance is
      about 36,000,000 miles. Its period is 88 days, and its
      diameter 3,000 miles.
      [1913 Webster]

   4. A carrier of tidings; a newsboy; a messenger; hence, also,
      a newspaper. --Sir J. Stephen. "The monthly Mercuries."
      --Macaulay.
      [1913 Webster]

   5. Sprightly or mercurial quality; spirit; mutability;
      fickleness. [Obs.]
      [1913 Webster]

            He was so full of mercury that he could not fix long
            in any friendship, or to any design.  --Bp. Burnet.
      [1913 Webster]

   6. (Bot.) A plant (Mercurialis annua), of the Spurge
      family, the leaves of which are sometimes used for
      spinach, in Europe.
      [1913 Webster]

   Note: The name is also applied, in the United States, to
         certain climbing plants, some of which are poisonous to
         the skin, esp. to the Rhus Toxicodendron, or poison
         ivy.
         [1913 Webster]

   Dog's mercury (Bot.), Mercurialis perennis, a perennial
      plant differing from Mercurialis annua by having the
      leaves sessile.

   English mercury (Bot.), a kind of goosefoot formerly used
      as a pot herb; -- called Good King Henry.

   Horn mercury (Min.), a mineral chloride of mercury, having
      a semitranslucent, hornlike appearance.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

boys-and-girls \boys-and-girls\ n. (Bot.)
   a Eurafrican annual (Mercurialis annua) naturalized in
   America as a weed; formerly dried for use as a purgative,
   diuretic or antisyphilitic.

   Syn: herb mercury, herbs mercury.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Mercurialis annua
    n 1: Eurafrican annual naturalized in America as a weed;
         formerly dried for use as a purgative, diuretic or
         antisyphilitic [syn: herb mercury, herbs mercury,
         boys-and-girls, Mercurialis annua]