1. the state or quality of being foolish; lack of understanding or sense.
2. a foolish action, practice, idea, etc.; absurdity.
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.
5. follies, a theatrical revue.
Etymology: from Old French folie, “madness”, from the adjective fol, “mad, insane”.
[Caitlin Hackett - Wisdom Wounded by Folly]