The Collaborative International Dictionary of English v.0.48:

Plentiful \Plen"ti*ful\, a.
   1. Containing plenty; copious; abundant; ample; as, a
      plentiful harvest; a plentiful supply of water.
      [1913 Webster]

   2. Yielding abundance; prolific; fruitful.
      [1913 Webster]

            If it be a long winter, it is commonly a more
            plentiful year.                       --Bacon.
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   3. Lavish; profuse; prodigal. [Obs.]
      [1913 Webster]

            He that is plentiful in expenses will hardly be
            preserved from                        --Bacon.
      [1913 Webster] -- Plen"ti*ful*ly, adv. --
      Plen"ti*ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plentifulness
    n 1: a full supply; "there was plenty of food for everyone"
         [syn: plenty, plentifulness, plenteousness,
         plenitude, plentitude]