The Collaborative International Dictionary of English v.0.48:

Forgetfulness \For*get"ful*ness\, n.
   1. The quality of being forgetful; prononess to let slip from
      the mind.
      [1913 Webster]

   2. Loss of remembrance or recollection; a ceasing to
      remember; oblivion.
      [1913 Webster]

            A sweet forgetfulness of human care.  --Pope.
      [1913 Webster]

   3. Failure to bear in mind; careless omission; inattention;
      as, forgetfulness of duty.

   Syn: Forgetfulnes, Oblivion.

   Usage: Forgetfulness is Anglo-Saxon, and oblivion is Latin.
          The former commonly has reference to persons, and
          marks a state of mind; the latter commonly has
          reference to things, and indicates a condition into
          which they are sunk. We blame a man for his
          forgetfulness; we speak of some old custom as buried
          in oblivion. But this discrimination is not strictly
          adhered to.
          [1913 Webster]

WordNet (r) 3.0 (2006):

forgetfulness
    n 1: tendency to forget
    2: unawareness caused by neglectful or heedless failure to
       remember; "his forgetfulness increased as he grew older"

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "forgetfulness":
   abandon, careless abandon, carelessness, casualness, cursoriness,
   disregardfulness, easiness, hastiness, heedlessness,
   inconsiderateness, inconsideration, indifference, insouciance,
   lack of foresight, laziness, lethe, oblivion, obliviousness,
   offhandedness, perfunctoriness, recklessness, regardlessness,
   sprezzatura, tactlessness, thoughtlessness, unaccommodatingness,
   unheedfulness, unhelpfulness, unmindfulness, unobligingness,
   unpreparedness, unreadiness, unsolicitousness, unsolicitude,
   unthinkingness, unthoughtfulness



The Devil's Dictionary (1881-1906):

FORGETFULNESS, n.  A gift of God bestowed upon doctors in compensation
for their destitution of conscience.