lead-pipe cinch
A certainty, an assured success. For example, "An engagement ain't always a lead-pipe cinch" (O. Henry, The Sphinx Apple, 1907). This colloquial expression is of disputed origin. It may allude to the cinch that tightly holds a horse's saddle in place, which can make it easier for the rider to succeed in a race; or it may allude to a cinch in plumbing, in which a lead pipe is fastened with a band of steel to another pipe or a fixture, making a very secure joint. [Late 1800s].