The point of rhyming slang was also to communicate secretly. Darby Kelly was a famous person real or not and it rhymed with belly. Once it was in use you could just say Darby and people knew what you meant. Google Harry Monk for some more fun. Darby Kelly was also used in another Harry Champion song soupity, soupity, soup. John Foreman always said that the songs were often about food because people didn’t have enough of it then. the real Darby Kelly was a drummer and the tune is based on a typical bugle call, simple but effective. Somerset fiddler William Winter had it in his manuscript book.