lead-pipe cinch

noun Slang.

1. an absolute certainty: It’s a lead-pipe cinch they’ll be there.
2. something very easy to accomplish: Getting him elected will be a 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]