A foolish act; an inconsiderate or thoughtless procedure; weak or light-minded conduct; foolery
If you say that a particular action or way of behaving is folly or a folly, you mean that it is foolish. It's sheer folly to build nuclear power stations in a country that has dozens of earthquakes every year
A folly is a small tower or other unusual building that is built as a decoration in a large garden or park, especially in Britain in former times. In architecture, an eccentric, generally nonfunctional (and often deliberately unfinished) structure erected to enhance a romantic landscape. Follies were particularly in vogue in England in the 18th and early 19th century. They might resemble medieval towers, ruined castles overgrown with vines, or crumbling Classical temples complete with fallen, eroded columns. In the U.S., the term has been applied to ornate gazebos. It may also be applied to any unusual building that is extravagant or whimsical in style