Tell me about cockney rhyming slang
Cockney rhyming slang is most closely associated with East London in England. It dates back to the 19th century. 

It works like this:

A word or phrase is replaced by another word or phrase that rhymes with it. For example instead of saying the word "face" you would say "boat race". The rhyming phrase is often longer than the original word and so the rhyme gets abbreviated. In our example "boat race" becomes "boat". 

So a sentence might become "you should have seen the look on their boat" instead of "you should have seen the look on their face". 

Another example might be "I'm off up the apple and pears to go to me Uncle Ned" - "I'm going upstairs to go to bed".