Definition of: folly

(fol′ē) noun plural ·lies
1. The condition or state of being foolish, or deficient in understanding; foolish conduct, idea, or act.
2. The result of a ruinous undertaking or enterprise, as a costly and useless structure left unfinished; also, any extravagant structure or undertaking, considered as showing the bad judgment of the originator.
3. An object of foolish or vicious attention or pursuit.
4. Immoral conduct; sin. See synonyms under IDIOCY. [<F folie <fol fool]