E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Arcadian.
A shepherd, a fancy farmer; so called because the Arcadians were a pastoral people, and hence pastoral poetry is called Arcadic.
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An Arcadian youth. A dunce or blockhead; so called because the Arcadians were the least intellectual of all the Greeks. Juvenal (vii. 160) uses the phrase Arcadcus juvnis for a stupid fool.