cockney monkey

[sipukka]

why is �500 referred to as a monkey?

[Quizmonster]

Some people claim it was based on an Indian 500-rupee banknote which had a picture of a monkey on it, but neither that nor any other source has ever been authenticated. The answer to your question, therefore, is really: "Nobody knows."

[Quizmonster]

Click http://www.worldwidewords.org/articles/money.htm for a website all about British money-terms.