The Shugborough Inscription is a set of letters, O U O S V A V V, found in the middle of the D and M etched into the Shepherd's Monument from the 18th century at Shugborough Hall in Staffordshire, England. It sits beneath an image of the "Shepherds of Arcadia" painting by Nicolas Poussin.

The meaning behind the letters of the Shugborough Inscription remains a mystery to this day, making it one of the most renowned unsolved ciphertexts in the world. The inscription gained notoriety after being mentioned in the 1982 book "The Holy Blood and the Holy Grail" by Michael Baigent, Richard Leigh, and Henry Lincoln.
The Monument At Shugborough
The Shepherd's Monument at Shugborough Hall, built between 1748 and 1763, was commissioned by Thomas Anson and financed by his brother, Admiral George Anson. The Flemish sculptor Pedro Scheemakers decorated the monument. The relief of Poussin's painting is set within an arch, with a rustic appearance, depicting a woman and three shepherds, two of whom are pointing at a grave.

The Latin text "ET IN ARCADIA EGO" which means "I am also in Arcadia" is carved on the tomb within the monument. The engraving of the tomb has a few modifications when compared to the original painting, including the addition of a sarcophagus in the main tomb.
Above Poussin's scene are two stone heads, the first one depicts a bald man smiling and the second one is goat-like horns resembling the Greek god Pan.

An unknown craftsman carved the mysterious letters O U O S V A V V on the relief carving of the monument, located between the letters D and M. In Roman tombs, the letters D M commonly stand for "Dis Manibus," meaning "dedicated to the shadows."

Theories Behind The Undeciphered Shugborough Inscription
Despite numerous theories put forward in recent years, staff at Shugborough Hall remain doubtful of any proposed solutions to the code. A spokesman for the National Trust-owned property stated in 2014, "We receive five or six claims of code-breaking each week, so we have become cautious of them."
Latin Initialism Theories
One theory is that the eight letters on the monument are a coded dedication by George Anson to his deceased wife. Oliver Stonor proposed in 1951 that the letters could be an acronym for "Optimae Uxoris Optimae Sororis Viduus Amantissimus Vovit Virtutibus" which translates to "Best of wives, Best of sisters, a most devoted Widower dedicates (this) to your virtues." This was the theory favored by former Bletchley Park employee Shiela Lawn, however, it has been criticized for being grammatically incorrect and not following Latin abbreviation rules.
Steve Regimbal suggests that the letters represent a Latin translation of the phrase "Vanity of vanities, saith the preacher; all is vanity" (Ecclesiastes 12:8) as "Orator Ut Omnia Sunt Vanitas Ait Vanitas Vanitatum." He theorizes that this phrase may be the origin of the inscription "OMNIA VANITAS" that was possibly carved on an alcove at the estate of George Lyttleton, an associate of Thomas Anson.
Keith Massey, a former NSA linguist, believes the letters to be an acronym for the Latin phrase "Oro Ut Omnes Sequantur Viam Ad Veram Vitam" which translates to "I pray that all may follow the Way to True Life." He suggests this is a reference to the Bible verse John 14:6, "Ego sum Via et Veritas et Vita" meaning "I am the Way, the Truth and the Life."
Cypher Theories
Dave Ramsden (2014) posits that the monument is a funerary altar dedicated to a syncretic female figure known as the "Shepherdess." He believes that the eight-letter inscription is a polyalphabetic cipher used to conceal the name "Magdalen."
George Edmunds in his book "Anson's Gold" (2016) proposed that the letters are a cipher for the latitude and longitude of an island where Admiral George Anson, Thomas Anson's brother, buried Spanish treasure. The treasure was located but never recovered due to unforeseen circumstances. Edmunds suggests that Anson received coded letters from the expedition leader containing part of the cipher.
English Initialism Theories
Countess of Lichfield suggested that the monument was built by Admiral Anson as a memorial to his wife. She believed the inscription refers to a poem about a Shepherdess named Alicia who converted pagans to Christianity. The initialism was thought to reference the line "Out Your Own Sweet Vale, Alicia, Vanishes Vanity. Twixt Deity and Man Thou, Shepherdess, The Way." However, no source for these words has been found.
A. J. Morton, a historian of the west coast of Scotland, noticed that some of the letters on the inscription match the names of residents of Shugborough in the early 19th century. He believes that the inscription reads "Orgreave United with Overley and Shugborough, Viscount Anson Venables Vernon."