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
    n 1: a former British gold coin worth 21 shillings
    2: (ethnic slur) offensive term for a person of Italian descent
       [syn: wop, dago, ginzo, Guinea, greaseball]
    3: a republic in western Africa on the Atlantic; formerly a
       French colony; achieved independence from France in 1958
       [syn: Guinea, Republic of Guinea, French Guinea]
    4: a west African bird having dark plumage mottled with white;
       native to Africa but raised for food in many parts of the
       world [syn: guinea fowl, guinea, Numida meleagris]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "guinea":
   bawbee, coin, crown, dollar, double eagle, doubloon, ducat, eagle,
   farthing, five-dollar gold piece, fiver, florin, fourpence,
   fourpenny, gold piece, groat, half crown, half dollar, half eagle,
   halfpenny, hard money, mag, meg, mite, moidore, monkey, napoleon,
   new pence, np, p, pence, penny, piece, piece of money,
   piece of silver, pony, pound, pound sovereign, quid, roll of coins,
   rouleau, shilling, sixpence, sovereign, specie,
   ten-dollar gold piece, tenner, threepence, threepenny bit,
   thrippence, tuppence, twenty-dollar gold piece, twopence