Polari is now particularly associated with gay culture, but it was widely used throughout theatrical circles and also among fairgrounds and circuses. It seems very likely that lorry drivers mingled with travelling showmen on the road and picked up some of their slang. Also, a number of Romany words have found their way into wider slang use, including "chav", "mush" and "cushty".

Whilst it is very likely that MacColl had come across Polari in the theatre, the whole point of the Radio Ballads was to use the language and voices of ordinary working people. I'm sure that he used the word because it was part of lorry drivers' vocabulary, otherwise why use a term from a different context, especially one that would not be familiar to many listeners?