We borrowed “cinch” from the Spanish cincha, a saddle-girth, used by the early settlers of our present West and Southwest.
And because the former girths of braided horsehair were so strong and could be drawn so tight that a rider could have no fear of a slipping saddle, “a cinch” became a synonym for a sure-fire certainty, something so assured that it could be considered easy.
But why “lead pipe” was introduced into the phrase some fifty or sixty years ago has me mystified.
An anonymous paragraph picked up a few years back says that a mythical cowboy, troubled by a bucking bronco that often snapped its cinch, throwing both saddle and rider, gave him the notion to make his cinch out of lead pipe.
But as one who has had experience, not only with horses but also with lead pipe, we can imagine few materials more productive of uncertainty than the latter.
Lead is far down the scale in tensile strength, and a cinch devised of lead pipe would be little more sturdy than cotton string.