E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Burying in a wall. The Vestal virgins among the Romans, and the nuns among the Roman Catholics, who broke their vows of chastity, were buried in a niche sufficiently large to contain their body with a small pittance of bread and water. The sentence of immuring was Vade in pace, or more correctly, Vade in pacem (Go into peacei.e. eternal rest). Some years ago a skelton, believed to be the remains of an immured nun, was discovered in the walls of Coldingham Abbey.
The immuring of Constance, a nun who had broken her vows, forms a leading incident in Scotts poem of Marmion.