Monkey is Cockney rhyming slang for £500.
This week’s post is unique in that, the word’s origin is lost to us. It is suggested that soldiers returning from India where, the 500 rupee banknote had a depiction of a monkey on it, returned to England and used the term to now refer to 500 pound sterling. However, there doesn’t seem to be a 500 rupee banknote showing a monkey; so at best it’s just speculation. Cockney rhyming slang is a lot like Who’s Line Is It Anyways, “where everything’s made up and the points don’t matter.”
Adding to last week’s example, here’s this week’s: