the state or quality of being foolish; lack of understanding or sense.
2.
a foolish action, practice, idea, etc.; absurdity: the folly of performing without a rehearsal.
3.
a costly and foolish undertaking; unwise investment or expenditure.
4.
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.
early 13c., from O.Fr. folie, from fol (see fool). Sense of "costly structure considered to have shown folly in the builder" is attested from 1650s. Used since M.E. of place names, especially country estates, as a form of O.Fr. folie in its meaning "delight." Meaning "glamorous
theatrical revue with lots of pretty girls" is from 1880, from French.