The whole company tries to figure out what Phutatorius is swearing about. They figured he was listening to the debate between Didius and Yorick, but nope.
Gastripheres, one of the scholars present, had ordered the kitchen to roast some chestnuts. When they were brought in, the scholars attacked them with such vigor that one fell directly into Phutatorius's fly … which he'd left open. Let that be a lesson to always zip (well, button) your fly.
It was an accident, maybe, but perhaps it was also a cosmic punishment for his filthy book, "How to Keep Mistresses." It's all about eighteenth-century karma. Anyway, Tristram says, it's his job to tell the facts, and the facts are that the chestnut fell into his fly.
At first, the heat actually felt kind of good—but pretty soon, it started to hurt. Not knowing about the chestnut, he thought that a newt might be biting him (really?)—hence the "Zounds!"
This has taken a long time to tell, Tristram says, but it all happened super quickly. Yorick picked up the chestnut, and Phutatorius figured that meant the chestnut was his. Everyone else thought so too, and they figured they knew why Yorick had attacked Phutatorius: Yorick thought his book about mistresses was a dud.
Yorick did no such thing, but people always did think the worst of him.