Folly is defined as (1) lack of good sense or of normal prudence, (2) inability or refusal to accept existing reality or to foresee inevitable consequences. Both of these definitions convey something of the policy stance that I associate with the term Keynesian.
Jim Buchanan‘s 1987 essay “Keynesian Follies,” reprinted in Vol. 1 of Jim’s Collected Works (pp. 164-178):