(as) fit as a butcher's dog

In excellent physical health. A butcher's dog would be well nourished from eating plentiful scraps of meat. Primarily heard in UK. I've seen that fella at the gym before. Trust me—he's as fit as a butcher's dog. My grandma has been rowing her whole life. She turns 87 this year and she's still fit as a butcher's dog. Yes, I did have surgery a few months ago, but I've completed my physical therapy and am as fit as a butcher's dog now.