Wordorigins.org

A cinch is an easy task or a gamble certain to pay off. A lead-pipe cinch is even more certain. These terms are ingrained in our vocabulary and we probably don’t think much about them. But if we do, they seem strange. How did a cinch, which is literally a saddle-belt, come to mean a certainty? And what the heck does lead-pipe have to do with it?

The first is easy enough. The literal meaning is North American in origin, coming from the Mexican Spanish cincha, meaning belt. It appears in English as early as 1866 in John K. Lord’s The Naturalist in Vancouver Island and British Columbia:

One girth only is used, styled a “synch,” made of horsehair.

Within a few years, the familiar spelling was in use in English. From Clarence King’s 1872 Mountaineering in the Sierra Nevada:

I leaned down and felt of [sic] the cinch, to see if it had slipped.

Since a cinch holds the saddle tight to the horse’s back, it’s only natural that the term would transfer to mean a tight hold, something that won’t slip, and even more figuratively, a certainty. And this 22 July 1888 citation from the New York World shows that the term did in fact quickly come to mean that:

The racehorse owner, who has a cinch bottled up for a particular race.

But what about the lead pipe? It dates to at least 1889, in the 17 December Decatur, Illinois Morning Review:

The money was transferred in a unique way. The briber and the bribed would sit down to a game of poker and “lead-pipe cinch” was nothing to the sure thing the legislators had.

Here’s another early example from the 14 October 1891 Salem Daily News (Ohio):

He Was a Dying Race Track "Tout" and He Imagined He Had "a Copper Lined Cinch" to Play on the Track That Day and No Time to Lose
[...]

The track will be heavy tomorrow, and I’ve got a copper riveted, lead pipe, copyrighted, air tight cinch. Firenze in the mud—she swims in it.

Exactly why lead pipe was chosen as a superlative is unknown. There is no known literal use of the term to refer to a specific method for applying a cinch to a horse and saddle. The term lead pipe cinch only applies to the metaphorical cinch, not the literal, equine one. Various explanations have been proffered about how lead pipes were used in tightening the horse’s cinch strap, but while some sound plausible, there is no evidence to support any of them.

(Sources: Historical Dictionary of American Slang; ADS-L; NewspaperArchive.com)