The phrase “cats out of the bag” comes from medieval markets where livestock was sold, then placed in bags for transport. Wiley merchants would tricks buyers after the livestock was selected by placing cats, which ran rampant, in the bags instead. When home, the unsuspecting buyer would realize the trick…and exclaim “the cat’s out of the bag!”
Murphy, Neal. “Loosing the Cat.” East Texas Serendipity: Discovering Stories, History, and Tales from an East Texan’s Perspective. N.p.: Xlibris Corporation, 2015. N. pag. Print.