Around the beginning of the 20th century this idiom, and its variations, started appearing in conversation and print. It's meaning is rather clear - even if I were dead I wouldn't be caught...engaging in that activity, wearing that item, doing that thing, saying that, engaging with that person, etc. ( source )

No origin is found for this popular phrase but its meaning is quite clear. ( source )

A ringer is a horse substituted for another of similar appearance in order to defraud the bookies. This word originated in the US horse-racing fraternity at the end of the 19th century. The word is defined for us in a copy of the Manitoba Free Press from October 1882: "A horse that is taken through the country and trotted under a false name and pedigree is called a 'ringer.'" And the addition of "dead" for the phrase, in this case, means "exact." Spot on mate.   Source So its meaning then, and now, is an exact duplicate. Like looking in the mirror.