folly (n.) Look up folly at Dictionary.com
early 13c., "mental weakness; unwise conduct" (in Middle English including wickedness, lewdness, madness), from Old French folie (12c.) "folly, madness, stupidity," from fol (see fool (n.)). Sense of "costly structure considered to have shown folly in the builder" is attested from 1650s. Used since Middle English of place names, especially country estates, as a form of Old French folie in its meaning "delight." Meaning "glamorous theatrical revue with lots of pretty girls" is from 1880, from French.