ET IN ARCADIA EGO
I have also lived / been in Arcadia.
This expression is used to show that the speaker has been happy during some time. It means that s/he has known the heavenly delights.

Arcadia was a central region of Peloponeso (Greece) full of beautiful valleys. The poets used to describe and praise Arcadia as a region of innocence and happiness.

ARCADIA - THE MYTH
This central region of Peloponeso, populated by shepherds, who adored the god Pan, and which was covered by dense forests, was, in the imagination of the old people, the mythical country of shepherd happiness. Arcadia was a kind of land paradise whose inhabitants, the arcadians, led a life wholly dedicated to music and singing.

The arcadism consisted of opposing material values, such as power and richness, to other more spiritual values, like love to nature, worship to beauty, the pleasure for music, etc.

Arcadia was named after Árcade o Arcadio, Zeus' (Jupiter's) son and the hunter nymph Calisto. Árcade taught his people how to grow wheat, how to produce bread and how to spin wool.

THE LEYEND OF CALISTO
Calisto was a forest nymph, who devoted herself to virginity, and spent her life in the forests, hunting, with the companion group of Ártemis (Diana). Zeus saw her and fell in love with her, uniting her in the figure of Ártemis, because Calisto avoided men. From their union Árcade was born.

Calisto was pregnant when Ártemis and her companions decided to have a bath in a spring; Calisto had to get undressed and her flaw was discovered. Indignant about Calisto's action, Ártemis threw her out and transformed her into a female bear. In the end, Calisto died, according to some, killed by Ártemis because she wasn't able to keep her virginity, and according to others, killed by Hera (Juno), because she was jealous of Calisto. Zeus transformed her into a constellation, the Great Bear.