If you Plug A Song or a Book then you promote it to get as many people as possible interested in it. Radio Normandie was one of the first independent radio stations. It was broadcast from Northern France and was also the first commercial radio station to transmit to England during the 1930s. One way they received income was via payments to play records and promote them throughout the country. The station's originator and foremost broadcaster was called Captain Leonard Plugge and this was probably how the phrase originated.