The phrase “dead right” and “dead to rights” are twin phrases in that they are nearly identical in use and meaning.
The word dead was used from the 16th century on to mean “utter, absolute, quite.” The “to rights” part of the phrase has been used since the 14th century to mean “in a proper manner” and later to mean “in proper condition or order.”
The implication of the phrase “dead right” or “dead to rights” was that every detail required by the law to make an arrest had been satisfied, making the arrest clean and justifiable. In other words, it was a “fair cop.”
The San Francisco newspaper, The City Argus, reported in an 1881 news story: “A man attempted to get into Banker Sather’s cash box and was caught ‘dead to rights‘ and now languishes in the city Bastille.”
The phrase dead right was commonly used by the police as early as 1919 to mean an individual committing a crime had been caught red-handed, as in: “Come clean! We have got you dead right!“
Historically Speaking 