After Sampson boasts that he’s “a pretty piece of flesh” (a fine specimen of a man), Gregory plays on the proverbial expression “neither flesh nor fish,” which means neither one thing or the other. He says that it’s a good thing Sampson is a piece of flesh and not a fish because if he were, he’d be a Poor John — a dried, shriveled-up fish sold in the fish markets, hardly something that would “stand” well.