Weakness or imbecility of mind; a foolish act; criminal weakness or depravity of mind; sin.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
A weak or absurd act; a weakness of mind; sin.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
Silliness; stupidity; foolish act.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)
Weakness of intellect; imbecility of mind; want of understanding; a weak or absurd act; an inconsiderate or thoughtless procedure; weak or light- minded conduct; criminal weakness; depravity of mind or actions; any object planned without its author having the means of bringing it to a successful completion, such as a magnificent mansion which exhausts a person's capital in building, or would ruin him in keeping up in proper style; as, “ We know indeed how this scorn will embody itself in a name given to the unfinished structure. It is called this or that man's folly; and the name of the foolish builder is thus kept alive for long after- years."- Trench.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
a stupid mistake
- Wordnet Dictionary DB (noun) (n.) (noun)
Want of understanding; foolishness; unbecoming conduct; criminal weakness; sin.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)

Usage examples:

Would the boy be killed for his folly
- Tabitha's Vacation by Ruth Alberta Brown
But it was a folly in itself.
- The Short Works of George Meredith by George Meredith Last Updated: March 7, 2009
Here wuz the beauty and dignity of the human form, onbroken by vanity and folly
- Samantha at the World's Fair by Marietta Holley