If you say that a particular action or way of behaving is folly or a folly, you mean that it is foolish.
例句
It's sheer folly to build nuclear power stations in a country that has dozens of earthquakes every year.在每年会发生几十次地震的国家建造核电站简直就是愚蠢至极。...a reminder of the follies of war.提醒人们战争之愚蠢的东西
N-COUNT
(尤指旧时英国花园等中的)装饰性建筑
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.