“A dead ringer” means an “exact duplicate” of someone or something. The phrase is used to refer to things that one cannot tell apart just by looking. To understand the origin of the phrase, we must first understand what the words “dead” and “ringer” mean in this context. Here, “dead” has the same meaning that it does in the phrase “dead on,” which means absolute or exact. It was originally used in marksmanship to mean that a shot was exactly on target. The term “ringer” comes from an old slang phrase “to ring” which meant “to switch, exchange or substitute.”

The phrase “dead ringer” was first used in horse racing in the US when unscrupulous horse owners would switch two horses that looked exactly the same in order to defraud bookies and buyers. For instance, a slow horse with long-shot odds could be switched with a faster one that resembled it, and the owner would bet and win big if the fake horse performed well.