folly - (noun) - 1: a lack of sense; foolishness; 2: a foolish action or belief

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Question:

When was a folly, not a folly?

Answer:

When it was a "Seward's Folly" (or "Seward's Icebox")

Alaska Purchase - Wikipedia

From the old French folie (madness) to modern French fol (fool or foolish) to middle English folly. 

From 1907 until the early 1930s the Ziegfeld Follies was a revue consisting of songs, skits, and dance numbers. 

The Folies Bergere began as the Folies Trevise in 1869 and became the Folies Bergere in 1872.  It was the most famous music hall in Paris and Josephine Baker made her debut there in 1926.  

The Las Vegas Les Folies Bergere operated in the Hotel Tropicana from 1959-2009.