Webster's English Dictionary

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folly

fol.ly \'fa:l-e-\ n [ME folie, fr. OF, fr. fol fool] 1: lack of good sense 
   or normal prudence and foresight  2: a foolish act or idea  obs  3a: EVIL, 
   WICKEDNESS; esp : LEWDNESS  3b: criminally or tragically foolish actions or 
   conduct  4: an excessively costly or unprofitable undertaking; esp : a 
   ruino usly costly often unfinished building