This expression means to be a very close duplicate, to have a strong resemblance for someone or something. A ringer was originally a counterfeit coin. If it made a ringing sound when dropped on a hard surface, it was a fake. Then it became a fast horse substituted for another of similar appearance with a bad racing record in order to defraud. 'Ring' is often used to mean 'exchange' or 'substitute'. 'Dead' here means exact or precise. The earliest record in print comes from 1888.

Übersetzung:

bis aufs Haar gleichen

Gegenteil(e)

The opposite of "to be a dead ringer" is "to be completely different".

Synonym(e):

You could also say: to be an exact counterpart.