The Answer:

To the best of our knowledge, the British pound has been called a "quid" both before and after Britain converted to a decimal money system in 1971. Its origin is uncertain, but it is believed that this term dates as far back as the Roman occupation of Britain (in the 1st to 5th century AD) and it may be derived from the Latin expression "quid pro quo," which means "one thing in return for another."

—The Editors