The Collaborative International Dictionary of English v.0.48:

Foolishness \Fool"ish*ness\, n.
   1. The quality of being foolish.
      [1913 Webster]

   2. A foolish practice; an absurdity.
      [1913 Webster]

            The preaching of the cross is to them that perish
            foolishness.                          --1 Cor. i.
                                                  18.
      [1913 Webster]

WordNet (r) 3.0 (2006):

foolishness
    n 1: the trait of acting stupidly or rashly [syn: folly,
         foolishness, unwiseness] [ant: wisdom, wiseness]
    2: the quality of being rash and foolish; "trying to drive
       through a blizzard is the height of folly"; "adjusting to an
       insane society is total foolishness" [syn: folly,
       foolishness, craziness, madness]
    3: a stupid mistake [syn: stupidity, betise, folly,
       foolishness, imbecility]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "foolishness":
   Urdummheit, absurdity, absurdness, big deal, blank mind,
   blankmindedness, blankness, brainlessness, bull, bunk,
   calm of mind, craziness, devilishness, devilment, devilry,
   deviltry, dottiness, elfishness, emptiness, emptiness of mind,
   empty-headedness, fallow mind, fatuity, flightiness, flimsiness,
   folly, frivolity, frivolousness, futility, giddiness, high spirits,
   idleness, ignorance, impishness, impracticality, imprudence,
   inanity, incapacity, indiscretion, ineptitude, injudiciousness,
   insanity, insensibility, irrationality, lackbrainedness,
   lackwittedness, levity, lightness, low IQ, ludicrousness, lunacy,
   mental blankness, mental weakness, mercuriality, mindlessness,
   mischief, mischievousness, nirvana, nonsense, nugacity, oblivion,
   passivity, playfulness, prankishness, pranksomeness,
   preposterousness, primal stupidity, puckishness, quietism,
   reasonlessness, ridiculousness, roguery, roguishness, scampishness,
   senselessness, shallowness, silliness, slackmindedness,
   slackwittedness, slenderness, slightness, sportiveness,
   superficiality, tabula rasa, thoughtfreeness, thoughtlessness,
   tranquillity, triflingness, triteness, triviality, trivialness,
   unintellectuality, unintelligence, unreasonableness, unwisdom,
   unwiseness, vacancy, vacuity, vanity, vapidity, volatility,
   waggery, waggishness, witlessness, youthful spirits