The Collaborative International Dictionary of English v.0.48:

Guinea \Guin"ea\ (g[i^]n"[-e]), n.
   1. A district on the west coast of Africa (formerly noted for
      its export of gold and slaves) after which the Guinea
      fowl, Guinea grass, Guinea peach, etc., are named.
      [1913 Webster]

   2. A gold coin of England current for twenty-one shillings
      sterling, or about five dollars, but not coined since the
      issue of sovereigns in 1817.
      [1913 Webster]

            The guinea, so called from the Guinea gold out of
            which it
            was first struck, was proclaimed in 1663, and to go
            for twenty shillings; but it never went for less
            than twenty-one shillings.            --Pinkerton.
      [1913 Webster]

   Guinea corn. (Bot.) See Durra.

   Guinea Current (Geog.), a current in the Atlantic Ocean
      setting southwardly into the Bay of Benin on the coast of
      Guinea.

   Guinea dropper one who cheats by dropping counterfeit
      guineas. [Obs.] --Gay.

   Guinea fowl, Guinea hen (Zool.), an African gallinaceous
      bird, of the genus Numida, allied to the pheasants. The
      common domesticated species (Numida meleagris), has a
      colored fleshy horn on each aide of the head, and is of a
      dark gray color, variegated with small white spots. The
      crested Guinea fowl (Numida cristata) is a finer
      species.

   Guinea grains (Bot.), grains of Paradise, or amomum. See
      Amomum.

   Guinea grass (Bot.), a tall strong forage grass (Panicum
      jumentorum) introduced. from Africa into the West Indies
      and Southern United States.

   Guinea-hen flower (Bot.), a liliaceous flower (Fritillaria
      Meleagris) with petals spotted like the feathers of the
      Guinea hen.

   Guinea peach. See under Peach.

   Guinea pepper (Bot.), the pods of the Xylopia aromatica,
      a tree of the order Anonace[ae], found in tropical West
      Africa. They are also sold under the name of Piper
      aethiopicum.

   Guinea plum (Bot.), the fruit of Parinarium excelsum, a
      large West African tree of the order Chrysobalane[ae],
      having a scarcely edible fruit somewhat resembling a plum,
      which is also called gray plum and rough-skin plum.

   Guinea worm (Zool.), a long and slender African nematoid
      worm (Filaria Medinensis) of a white color. It lives in
      the cellular tissue of man, beneath the skin, and produces
      painful sores.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Guinea worm
    n 1: a painful and debilitating infestation contracted by
         drinking stagnant water contaminated with Guinea worm
         larvae that can mature inside a human's abdomen until the
         worm emerges through a painful blister in the person's skin
         [syn: Guinea worm disease, Guinea worm,
         dracunculiasis]
    2: parasitic roundworm of India and Africa that lives in the
       abdomen or beneath the skin of humans and other vertebrates
       [syn: Guinea worm, Dracunculus medinensis]