folly
n. C / Un. a very foolish action or a lack of good sense. It can also mean a decorative building in a garden that has no real purpose.
n. a lack of good sense or foresight; a foolish action, practice, or idea. In an architectural context, refers to a costly ornamental building with no practical purpose, often built in a large garden.
It would be sheer folly to go out in this storm.
The king's decision to invade the neighboring country proved to be a costly folly that drained the national treasury.
The hillside is dotted with eighteenth-century follies, including a miniature Gothic tower that serves no purpose other than to provide a picturesque focal point for the estate's visitors.
Uncountable when referring to the abstract quality of foolishness; countable when referring to a specific act or a decorative building.