E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
From pecus, cattle, especially sheep. Varo says that sheep were the ancient medium of barter and standard of value. Ancient coin was marked with the image of an ox or sheep. We have the Gold Sheep (mouton dor) and Gold Lamb (agneau dor) of ancient France, so called from the figure struck on them, and worth about a shilling. (Latin, pecuniarius, pecunia.)