The phrase "go to Putney on a pig" may seem nonsensical, but it can be used as a fun way to express the idea of embarking on an unlikely or impossible task. If you're looking for synonyms, you could try phrases like "climb Mount Everest on a pogo stick," "build a rocket ship out of toothpicks," or "swim across the Atlantic Ocean with lead boots." These phrases are all similarly whimsical and playful, and can be used in a variety of situations to convey a lighthearted and humorous tone. So the next time you want to express an unlikely idea, try out one of these synonyms for "go to Putney on a pig" and see what kind of reaction you get!