This phrase doesn’t seem to be proper English (though I could be wrong, it just sounds funny to me.) What exactly does it mean and where did it come from? A search returned no matches.
“To turn state’s evidence” means:
From A layman’s guide to legal terms.
In other words, testifying not in support of the defendant’s evidence, but in support of the state, or “turning” to the state’s side.
IANAL, but from what I understand, it means that someone accused of being connected with a crime volunteers to “turn” on his cronies and supply the “State” with “evidence” about the larger fish in the pond.
For example, Vinnie the Squealer is arrested for operating an illegal dog grooming salon in his basement. The DA is sure Vinnie is connected with Big Patsy the Chef, who they are certain runs all the numbers out of Brooklyn, but whom they can’t get enough evidence to build a case against.
The DA tells Vinnie that instead of doing hard time (he’s a two-time loser in the illegal dog grooming scam), things will go easy for him if he testifies against Big Patsy.
Thus, Vinnie has turned states evidence to put the finger on Big Patsy and avoid going up the river to the big house for illegal dog grooming.
It will help if you read this out loud, out of the corner of your mouth, while flipping a silver dollar and shifting your eyes around the room. Make sure you’re wearing a pin-striped suit, a fedora, and a shoulder holster.
Ok. I was pretty certain of what it meant but now I’m completely sure. I guess the phrase itself just sounds really funny and improper to me because I never really hear it outside of the news or crime dramas.
The OED cites “to turn evidence” in England from 1722. It went on in the 19th century to be called “Queen’s evidence.” Later “King’s” and “State’s.”
Mathews cites it as early as 1927 in the US with the meaning of informing on an accomplice to lessen your own punishment.
Should have previewed! It is cited from 1827 in US.
Hahaha!!! That’s why I love this place.
If you’ve seen the movie Goodfellas, Henry Hill is a perfect example of a snitc…err, turning “states evidence”. Instead of facing 20 to life for narcotics trafficking, he was granted immunity for testifying against other crime lords. (The guys played by Robert De Niro and Paul Sorvino…forget their names at the moment but they were changed in the movie anyway.)
Monica Lewinsky was also granted immunity in the whole Clinton impeachment debacle. She would have had to testify, but they never called her.
BTW, I always wondered, what happens if two crime partners turn states evidence on each other? Like John Muhammed Williams and John Malvo…
Won’t happen- once one partner takes the deal (testify in exchange for immunity), the other won’t get it.
DAVEW0071, a perfect explanation. But given the scenario you’ve written, you should have phrased the whole thing in the present tense. (And I assume you understand why! ;))
My definition of that phrase would be, “you’re in big trouble either way!” 
You might not do jail time but somebody’s gonna get ya.
Sorry for that little aside.
That, by the way, would an example of a possible scenario in the Prisoner’s Dilemna.
Well, you can squeal like a pig but if the state feels it can get a conviction without your testimony, you can’t force them to give you a deal. In the case of Williams/Malvo, Williams can offer to give a detailed account of every shooting and a district attorney could just declare “We have your car and your rifle, so we’re going to execute you with or without your testimony.”
My failure to employ “Runyonics” to my answer distresses me no little and more than somewhat, Polycarp. I take your mild rap on the knuckles with all due humility, as I am someone who wishes at all times to take instruction from a citizen such as yourself, who is known to one and all as a guy who is apt to appreciate the finer points of the Broadway argot.
And, KGS, as amused as you may be with the idea of Vinnie the Squealer running an illegal dog grooming salon out of his basement, there was a story in New York recently about a guy who was treating pets for all sorts of things, I believe cash only, really cut-rate, out of his apartment. He had no veterinary credentials at all. Fortunately for the animals, he was arrested.
And I would have gotten away with it, too, if it hadn’t been for you meddling kids and that dog!