Approver

a person who .
Old English Law. an accomplice to a felony who confesses his or her guilt and gives evidence against his or her confederates.
Historical Examples

Monk was with his troops in Scotland, but had declared himself an approver of the proceedings of the Parliament.
The Cavalier Songs and Ballads of England Various

If ye will have God author and approver of your reformation, ye must not follow their footsteps.
The History of the Reformation of Religion in Scotland John Knox

“Nay, if your greatness requires more proof, I can give you some now,” continued the approver.
Confessions of a Thug Philip Meadows Taylor

I would have turned “approver” against my father rather than gone on in this fashion.
The Dodd Family Abroad, Vol. I.(of II) Charles James Lever

The statement of the approver confirmed the declarations of the other witnesses, and both prisoners were found guilty.
The Chronicles of Crime or The New Newgate Calendar. v. 2/2 Camden Pelham

Oates was the principal witness against them; but he was followed by Praunce, an approver, and others.
The Chronicles of Newgate, vol. 1/2 Arthur Griffiths

Miswritten as approver in the seventeenth century, though distinct from approve (from approbare).
Chaucer’s Works, Volume 5 (of 7) — Notes to the Canterbury Tales Geoffrey Chaucer

I would rather pay for every breakfast, and even every dinner, eaten by me since here I came, than take money as an approver.
Lorna Doone R. D. Blackmore

Certainly, he said; that is what the approver of injustice says.
The Republic Plato

As a counsellor she was not wanted; but as an approver, (a much safer character,) she was truly welcome.
Emma Jane Austen