the state or quality of being foolish; lack of understanding or sense.
a foolish action, practice, idea, etc.; absurdity: the folly of performing without a rehearsal.
a costly and foolish undertaking; unwise investment or expenditure.
Architecture. a whimsical or extravagant structure built to serve as a conversation piece, lend interest to a view, commemorate a person or event, etc.: found especially in England in the 18th century.