Q: I have never been able to parse the expression “turn state’s evidence.” Does the witness turn himself into evidence for the state, or turn over evidence to the state?
A: A convicted or accused criminal, as you know, “turns state’s evidence” by testifying in court against former accomplices.
Why “turn” evidence? We don’t know, though we suspect that the usage may have been influenced by both the “turn over” and “turn against” senses of the verb “turn.”
In fact, “turning state’s evidence” indicates both turning against accomplices and turning over evidence.
When the courtroom expression showed up in writing in the early 1700s, it was simply to “turn evidence.”
The earliest example in the Oxford English Dictionary is from Colonel Jack, a 1723 novel by Daniel Defoe: “One of the Gang, who to save his own Life, has turn’d Evidence.”
In modern British usage, the expression now refers to “King’s” or “Queen’s” evidence, as in this citation from The Hillyars and the Burtons, an 1865 novel by Henry Kingsley: “I hate a convict who turns Queen’s evidence.”
In the US, such testimony is called “state’s evidence,” as in this OED example from a Dec. 24, 1886, issue of Science: “Mr. Bartlett Channing Paine comes into court, and, as state’s evidence, gives the following testimony.”
Finally, here’s a figurative “King’s” example from the Dec. 25, 1889, issue of the Daily News in London: “The Bishop might have been better employed than in turning King’s evidence against the Sermon on the Mount.”
When the verb “turn” showed up in late Old English, it meant to “cause to move round on an axis or about a centre; to cause to rotate or revolve, as a wheel,” according to the OED.
The dictionary’s earliest citation is from Ælfric’s Lives of the Saints, believed to have been written in the late 990s: “Þa tyrndon þa hæðenan hetelice þæt hweowl” (“then the bloodthirsty heathens turned the wheel savagely”).
By the 1300s, people were using the verb in the expressions “turn against” (change from friend to foe) and “turn one’s back” (abandon someone or something).
Here’s an OED example, dated around 1300, from Thomas Wright’s Political Songs of England (1839): “turnden hem aȝeynes with suerd ant with launce” (“turned against them with sword and with lance”).
And here are a couple of later “turn one’s back” examples from Shakespeare:
“The shame Of those that turnd their backes” (from Henry IV, Part 2, 1600) … “To turne thy hated backe Vpon our kingdome” (from King Lear, 1608).
In the 1500s, people began using the expression “turn one’s coat” to mean change one’s principles or party.
The term “turncoat,” which is more common today, showed up soon after. Here’s an example from a 1570 church history by John Fox:
“I will beleue none of you all, for you be turne coates, and chaungelinges, and be wauering minded.”
Around the same time, the expression “turn over” came to mean to transfer or hand over. The first OED example is from Richard Huloet’s 1552 dictionary, Abcedarium Anglo Latinum: “Turne ouer, transuerto.”
from Grammarphobia https://www.grammarphobia.com/blog/2017/07/turn-states-evidence.html