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].

An easy type of joint used in fabricating pipe from elongated sheets of lead. First, the lead sheets are formed in a cylindrical shape with a flat overlap perpendicular to the cylinder; then, the flat overlap is folded over and crimped, thereby forming a sealed joint.