Term used for the fineness of coins. Once thought to be so called because some coins had a star; another suggested derivation was from *Easterlings, who were coiners brought to England by Henry II to improve the currency. Foreign copies of the English *penny known as 'esterling' were esp. common in the late 13c. In 1213, when King John swore fealty to the pope, he also swore to pay for ever mille marcas sterlingorum = 1,000 marks of sterling. Now, however, it is suggested sterling is derived from OldEngl. steor = firm.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.