I'm going to take a shot at this. Take it with a grain of salt. All these people British or Cornish, Celtic or Cymric, engaged in word play of all sorts. Rhyming slang, double meaning, etc . . . they're singing in a language not their own, English. A lot of phonetics are in play. Not saying they were illiterate. Often people back in the day were more literate than people now, in many way. But you don't have to read music to come up with a song anymore than you have to be Shakespeare to write a poem. If you listen to a child pronounce a word without knowing the spelling it comes out a bit funny sometimes. Same as when Japanese borrow foreign words. Sometimes you have to get a little creative to get what the original word was. Remember even the Irish in the film Snatch had trouble understanding Brad Pitt's Pikey character. If you've ever hung out drinking with someone with a strong accent of any kind you know what I mean. So it isn't much of a stretch to turn 'Work For The Devil' into 'Whack Fal the Diddle'. Drink a few whiskeys, and "Must Have" quickly become "Musta" and finally "Musha". I'm not saying that's what it is. Remember when writing a song it doesn't matter what it means to anyone but the author, as long as it sounds good. The Melvins write unintelligible lyrics all the time. Maybe it means something to them, but they probably prefer that it isn't understood. John Lennon found out college professors had students analyze Beatle songs, so he put in more nonsense than usual, that's how you get I Am The Walrus. Could be a song about someone's mustache. Out of Boyo you get Boy. Someone said, Daddy-O means Grandfather. Makes sense to me, since O, just like Mc or Mac means Of, or Son Of. Here's the question... does he mean his real Grandfather, or Grandfather in a kindly was, or just some old fool? Wrap your head around that one. And then drink a couple of whiskeys and try to figure out the lyrics from a writer's perspective while switching between Gaelic, Brythonic & Alcoholic. You'll find all sorts of new meanings! ; )