Arcadia was a region of the Peloponnese in Ancient Greece, the home of simple herdsmen who worshipped the god Pan and sang to the accompaniment of pipe music. Over time, various poets (including Virgil in his 'Bucolics') began to depict Arcadia as a rural paradise inhabited by idealized shepherds, a theme which was repeated in many versions of pastoral poetry.
'Et in Arcadia Ego' is the title of two paintings by Nicolas Poussin (1594-1665). In the later of the two paintings (also known as 'The Arcadian Shepherds'), shepherds are gathered around a tomb on which the Latin words are inscribed. They may be translated as, 'And I too am in Arcadia' (where 'I' is the personification of death) or as 'I also used to live in Arcadia' (where 'I' refers to the person in the tomb). Either way, the words may be taken to imply that 'in the midst of life we are in death'*, and the painting may be seen as an example of memento mori, works of art designed to remind us of our mortality.
Later in Brideshead Revisited, we learn that Charles Ryder has a memento mori, namely a skull with Et in Arcadia ego inscribed on its forehead. The quotation is particularly relevant to the theme of 'Book One' in that the characters do find an 'earthly paradise', but one whose duration is short-lived.
* Words of the burial service in the Book of Common Prayer.
Listen on Spotify:
and a musical memento mori... Danse Macabre by Saint-Saens