I think it's just the English language evolving, and people are/have always been, lazy in their speech, shortening words and mispronunciation. Slang appeared and we have the Cockney Rhyming Slang amongst others. My answer is the trimmings is the other food which traditionaly accompany the Christmas bird. That would include stuffing, gravy, sauces and from the north,Yorkshire pudding. No replies to that one telling me my history info is wrong, because it just doesn't matter.