I have no explanation for this shit. I don't understand it, it's crazy, and Brits just accept it and use it as if it's normal to rhyme words with other words that have no related meaning and use that new word to stand for what they wanted to say. GOT THAT? The apparent origins of Cockney slang were shysters trying to confuse police in the pub, and talk about their scams in code, but their wordplay has entered British lexicon and it's completely incomprehensible to foreigners.
Try to understand it, if you can, by reading this.