The Oxford Thesaurus An A-Z Dictionary of Synonyms - folly n.

1) foolishness, nonsense, absurdity, daftness, silliness, preposterousness, absurdness, senselessness, fatuousness, fatuity, rashness, stupidity, asininity, inanity, nonsensicalness, idiocy, imbecility, irrationality, lunacy, insanity, madness, craziness, eccentricity, weak-mindedness, feeble-mindedness, simple-mindedness, muddle-headedness, thickheadedness, stolidity, stolidness, obtuseness, brainlessness, Colloq dumbness, dopiness, nuttiness, US and Canadian kookiness: They saw the folly of fighting against such enormous odds. It would be folly to leave in this storm. 2) absurdity, mistake, blunder, error, faux pas, gaffe, Colloq goof, fluff: What follies have been committed in the name of honour!