1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): a lead-pipe cinch
Fig. something very easy to do; something entirely certain to happen. • I knew it was a leadpie cinch that I would be selected to head the publication committee.
McGrawhill's American Idioms And Phrasal Verbs