1 a : (noncount) : the lack of good sense or judgment : FOOLISHNESS
the folly of driving fast on steep, winding roads
his folly in thinking that he would not be noticed
The folly of such an action should be apparent to everyone.
b : a foolish act or idea : foolish behavior
(noncount)
Their pranks were just youthful folly. (=the kinds of foolish things that young people often do)
(count)
engaging in youthful follies
the follies of the modern world
2 : (count) : a very unusual or fancy building that was built in a garden for decoration or amusement in the past