To turn King's evidence

to confess a crime and give evidence against one's accomplices.

See also: Evidence

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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References in classic literature ?
Damme if I think he meant to turn king's evidence; but he's that sort of bragging fellow, the bragging runs over hedge and ditch with him, till he'd brag of a spavin as if it 'ud fetch money.
Then, in a remarkable development on 8 March 1831, Michael Mulvay expressed to the gaol chaplain that his brothers had been responsible for the shooting of Burns and he wished to turn King's Evidence against them.