Folly (noun)

1. Lack of good sense; foolishness.
2. A costly and foolish undertaking.
3. An ornamental building constructed to give pleasure or to serve as a conversation piece.

Origin:

Middle english: from old french folie, from latin follis "bellows, windbag" (later also "folly"); the original sense was "worthless thing", influenced by fol "fool".

Examples:

1. It was a great folly to invest so much money in such a risky venture.
2. The new emperor"s first act was to build a palace, which was a folly given the state of the empire"s finances.
3. The folly was a popular spot for tourists to visit and take pictures.
4. It was a great folly to leave the keys in the car with the windows open.
5. He realized too late that his decision to quit his job was a folly.