E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A corruption of Giacomo-postolo (James the Apostle). So called after his relics were transferred thither from Iria Flavia (El Padron) on the borders of Galicia, in the ninth century. Leo III. transferred the See of Iria Flavia to Compostella. (Somewhere between 810 and 816.)