Folly is foolishness or a lack of good sense, especially when someone does something obviously unwise that they should have known better than to do. When a king spends all his kingdom's money building a golden palace while his people go hungry, that's folly. When someone ignores all warnings and walks onto thin ice, that's folly too.

The word suggests that the person could have seen the problem coming if they'd stopped to think, and often involves ignoring obvious warnings or consequences. If you stay up until midnight playing video games when you have an important test the next morning, that's folly, not just bad luck. If a town builds houses right on a cliff edge that's clearly eroding into the sea, that's folly.

Folly often carries a sense of regret or consequences. People sometimes build expensive structures that turn out to be useless or impractical, and these buildings are called follies. In England, wealthy landowners once built decorative ruins and fake temples on their estates just for show. These elaborate but pointless structures became known as follies because they looked impressive but served no real purpose.

The phrase “the folly of youth” refers to the foolish decisions young people sometimes make before they've learned from experience. Everyone commits some folly in their lives, but wisdom means learning from those mistakes instead of repeating them.