The phrase "dead ringer" means an exact duplicate or a very close resemblance to someone or something else. Its origin is rooted in horse racing and the language of the 19th century. 

The phrase "dead ringer" comes from the world of horse racing in the late 19th century. In this context, a "ringer" was a horse that was substituted for another horse, usually one of lesser ability, in order to deceive and improve the odds for betting purposes. 

The term "dead" in this phrase means "exact" or "precise," emphasizing that the substituted horse was an exact duplicate in appearance of the original. The term "ringer" itself was derived from the verb "ring," which meant to exchange or substitute. 

This usage can be traced back to the practice of swapping out horses in races. The earliest known use of "dead ringer" in print dates back to the late 19th century in American newspapers, particularly in relation to horse racing scams. 

Over time, the term evolved from its specific use in racing to a broader application, referring to any close or exact duplicate.