Spelmab derives the word from esterlings, merchants of the Hanse Towns, who came over and reformed our coin in the reign of John. Others say it is starling (little star), in allusion to a star impressed curbe coin. Others refer it to Stirling Castle in Scotland, where money was coined in the reign of Edward I.
(Sir Matthew Hale.)
“In the time of King Richard I., monie coined in the east parts of Germany began to be of especiall request in England for the puritie thereof, and was called Easterling monie, as all the inhabitants of those parts were called Easterlings: and shortly after some of that countrie, skillfull in mint matters and allaies, were seat for into this realm to bring the coine to perfection, which since that time was called of them sterling for Easterling.” —Camden.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894