UK [ˈfɒlɪ] / US [ˈfɑlɪ] noun
Word forms "folly":
singular folly plural follies
1) [countable/uncountable] formal a way of thinking or behaving that is stupid and careless, and likely to have bad results

The judge described the incident as an act of folly.

it is folly to do something:

It is absolute folly to go mountain climbing without the proper equipment.

the folly of (doing) something:

She soon realized the folly of her actions.

I pointed out the folly of overspending on that scale.

a) [countable] a building that has no practical use and is built as a decoration
b) mainly journalism a building or project that costs a lot of money and is not useful

English dictionary. 2014.