Ever been watching an old detective movie and heard the phrase, "She's a dead ringer..." The phrase is intended to mean that there is an uncanny match or similarity between two things. For instance, you might say that Mick Jagger is a dead ringer for a batfish.
The phrase originates in the mid to late 19th century and comes from the world of horse racing. It seems that in the old days of racing, it was common to replace a well known race horse with a horse that was very similar in appearance. This was done to defraud the bookies at the races. The word is defined for us in a copy of the Manitoba Free Press from October 1882:
The word "dead" was later added to the phrase as part of American popular language. For example, we often say "dead on" or "dead set." On of the other uses of the word dead is to express a level of "exactness," if you will."A horse that is taken through the country and trotted under a false name and pedigree is called a 'ringer.'"
So, there it is. An interesting explanation to a commonly used phrase. If you didn't find it interesting, maybe you will enjoy learning that Meatloaf had an album called "Dead Ringer." Here is the album cover: