(Psalm li. 1). “Have mercy upon me, O God, according to They lovingkindness: according unto the multitude of Thy tender mercies blot out my transgressions.” This verse was so called because it was the trial-verse of those who claimed benefit of clergy; and if they could read it, the ordinary of Newgate said,
“Legit ut clericus, ” and the convict saved his neck, being only burnt in the hand and set at liberty.
If a clerk had been taken For stealing of bacon. For burglary, murder, or rape. If he could but rehearse (Well prompt) his neck-verse, He never could fail to escape.
British Apollo (1710).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Neck-verse from Infoplease: