OK, I've solved this one: there is a sign in British Sign Language in which the pointed forefinger of each hand passes simultaneously over the signer's head from front to back. It is used to indicate a total failure to understand what is being said. It is often glossed as 'that went right over my head', but among the Deaf it is pronounced (and I don't have time to explain how a sign can be pronounced - you'll just have to take my word) POW!
Simple then. Deaf person turned up to early song session with interpreter. Someone started to sing Dainty Davey. Interpreter tried to translate 'Leaze me on your curly...', but then realized it made no sense and went POW! And so it passed into the Tradition. Well, that's my story.