The Collaborative International Dictionary of English v.0.48:

Folly \Fol"ly\, n.; pl. Follies. [OE. folie, foli, F. folie,
   fr. fol, fou, foolish, mad. See Fool.]
   1. The state of being foolish; want of good sense; levity,
      weakness, or derangement of mind.
      [1913 Webster]

   2. A foolish act; an inconsiderate or thoughtless procedure;
      weak or light-minded conduct; foolery.
      [1913 Webster]

            What folly 'tis to hazard life for ill. --Shak.
      [1913 Webster]

   3. Scandalous crime; sin; specifically, as applied to a
      woman, wantonness.
      [1913 Webster]

            [Achan] wrought folly in Israel.      --Josh. vii.
                                                  15.
      [1913 Webster]

            When lovely woman stoops to folly.    --Goldsmith.
      [1913 Webster]

   4. The result of a foolish action or enterprise.
      [1913 Webster]

            It is called this man's or that man's "folly," and
            name of the foolish builder is thus kept alive for
            long after years.                     --Trench.
      [1913 Webster]

WordNet (r) 3.0 (2006):

folly
    n 1: the trait of acting stupidly or rashly [syn: folly,
         foolishness, unwiseness] [ant: wisdom, wiseness]
    2: a stupid mistake [syn: stupidity, betise, folly,
       foolishness, imbecility]
    3: 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]
    4: foolish or senseless behavior [syn: folly, foolery,
       tomfoolery, craziness, lunacy, indulgence]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "folly":
   absurdity, absurdness, act of folly, asininity, battiness, blunder,
   brainlessness, buffoonery, clownishness, crackpottedness,
   crankiness, craziness, daffiness, daftness, desipience, dottiness,
   dumb trick, dumbness, eccentricity, error, fatuity, fatuousness,
   faux pas, fluff, foolery, foolheadedness, foolishness, frivolity,
   frivolousness, gaffe, giddiness, goof, goofiness, idiocy,
   imbecility, imprudence, inanity, indiscretion, ineptitude,
   insanity, irrationality, kookiness, lunacy, madness, mindlessness,
   mistake, niaiserie, nonsense, nugacity, nuttiness, obtuseness,
   preposterousness, queerness, rashness, sappiness, screwiness,
   senselessness, silliness, sottise, stolidity, stolidness,
   stupid thing, stupidity, thoughtlessness, triflingness, triviality,
   unwise step, wackiness, weak-mindedness, weirdness, witlessness,
   zaniness, zanyism



The Devil's Dictionary (1881-1906):

FOLLY, n.  That "gift and faculty divine" whose creative and
controlling energy inspires Man's mind, guides his actions and adorns
his life.

    Folly! although Erasmus praised thee once
        In a thick volume, and all authors known,
        If not thy glory yet thy power have shown,
    Deign to take homage from thy son who hunts
    Through all thy maze his brothers, fool and dunce,
        To mend their lives and to sustain his own,
        However feebly be his arrows thrown,

    Howe'er each hide the flying weapons blunts.
    All-Father Folly! be it mine to raise,
        With lusty lung, here on his western strand
        With all thine offspring thronged from every land,
    Thyself inspiring me, the song of praise.
    And if too weak, I'll hire, to help me bawl,
    Dick Watson Gilder, gravest of us all.
                                                     Aramis Loto Frope