monkey

[mfcmfc]

why is �500 known as a monkey

[feebee]

Googled and found this:

Perhaps the oldest and best established [British slang terms] are the pony and the monkey, respectively �25 and �500. Another term for �25 is macaroni, rhyming slang for pony, which was invented in the 19th century. [Another source claims the term monkey came from soldiers returning from India, where the 500 rupee note had a picture of a monkey on it. They used the term monkey for 500 rupees and on returning to England the saying was converted for sterling to mean �500.]

[Quizmonster]

Feebee's answer above re Indian notes is the one usually offered for the source of 'monkey', but the plain fact is that no one actually knows. The Google site referred to above seems to be based on the words of Michael Quinion, a noted etymologist and lexicographer, so here's what he actually has to say about it...
"Of all the money slang terms, perhaps the oldest and best established are the pony and the monkey, respectively �25 and �500. Of the two, pony seems to be the older, being documented from the end of the eighteenth century; monkey is first recorded in 1832 (though the German visitor who did so thought it was only five pounds, which might have been a bit of a shock for him when he found out). As usual, nobody seems to know where the words originated, except that they both grew out of gambling, the race track and general raffishness. Another term for �25 is macaroni, rhyming slang for pony, which was invented last century and is still to be heard in some specialist fields." (My underlining)
As you can see, he does not even mention Indian currency and nor does The Oxford English Dictionary. So, the real, answer to your question, Mfcmfc, is "Nobody knows."

[mrsbob]

mfc - do you by any chance support Millwall? Surely you should know the answer coming from that part of London!!! Having said that, my husband supports Millwall, comes from Pekham and has no idea!