An absolute certainty. This cliché dates from the first half of the nineteenth century and originally alluded to a bet one could not lose. Appropriately, Jane Smiley used it in her racetrack novel, Horse Heaven (2000): “‘Curtis, you’ve been around the racetrack for twenty-five years or more. Don’t you know that the only sure thing is that a sure thing is never a sure thing?’” Without the article, sure thing also is a reply that means “Yes,” or “Certainly.” This usage dates from the late 1800s.