Dinky doo is rhyming slang for twenty-two in London Cockney. It originated as a nickname for the pinball machine called a Dinkey Dee, which was released in 1962 by Bally Manufacturing Company. The term came from the sound that the machine made when it was played. Other theories exist about how this name became associated with 20-22s. Some say it started in the 1960s when there were large numbers of Americans living in London and that they often took advantage of the free entertainment at night by playing pinball machines in pubs. They would be likely to call them dinkies because 22 is a common number among American friends. Others believe it began much earlier than that and says that the sound that the pinball makes when it's being played is what led to its becoming a popular term for 20-22s. This theory is supported by the fact that the Dinkey Dee was first released in 1962 which means that it was already popular when 20-22s started calling themselfes dinkies. Yet another theory says that it originated as a reference to the fact that when you hit a pinball machine for the first time it makes a very small noise known as a "dink".