lead-pipe cinch

A task that is easily, effortlessly, or certain to be accomplished. I've been running marathons for years now, so this 5K run will be a lead-pipe cinch for me. Everyone assumed her election would be a lead-pipe cinch, so it shocked the entire nation when she lost.

Fig. something very easy to do; something entirely certain to happen. I knew it was a lead-pie cinch that I would be selected to head the publication committee.

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]

An absolute certainty; an easy success. Unlike the meaning of this cliché, the etymology is uncertain. It originated in America in the late nineteenth century and may refer to (l) the cinch that holds a horse’s saddle in place, which, if well fastened, makes it easier for the rider to win a race; or (2), more likely, to plumbing, where a lead pipe is fastened with a steel band to another pipe or fixture, making for a very secure joint. O. Henry used the term in a short story published in 1907 (The Sphinx Apple): “An engagement ain’t always a lead-pipe cinch.”