'arcadian' originates from English via Latin and Greek, ultimately from Greek 'Αρκαδία' (Arkadía), where 'Αρκαδία' referred to the region Arcadia in the Peloponnese.

'Αρκαδία' (Greek, the place-name) passed into Latin as 'Arcadia', then into English (as the place-name and literary concept), and the adjective 'arcadian' developed in English to mean 'of or relating to Arcadia' and, by extension, 'pastoral, idyllic'.

Initially it meant 'of or from Arcadia' (a geographic/demonymic sense); over time it evolved into the broader sense 'idealized rural or pastoral simplicity; idyllic'.