When a gentleman said he bet a monkey, what did he mean?
500 pounds, of course.
A gentleman close to the River Tick might have only afforded a pony (25 pounds sterling…which later, through rhyming slang, became macaroni).
Although no one is quite sure where the term monkey (in reference to $500) came from, there is some speculative consensus that it was brought back to the British Isles from India, where the 500 rupee note had a picture of a monkey on it (and the 25 note a pony). Since this sort of slang would’ve been used primarily by nabobs and gents when making bets, it makes sense…and somehow money is often fondly reverted to slang (think modern day hund-o, 5 large, Gs, buck, green, etc).
Here is some more delightful monetary slang:
Regency Reader 