(slang) a doubly sure or a doubly easy thing. The
phrase derives from a trick used in tightening down the
cinch (saddle strap) on an uncooperative horse, employing a so-called
lead pipe to
turn the strap like a tourniquet. After the strap was cinched tight, the
pipe was then tucked between the strap and the horse's belly, "double-cinching" the horse.