The punchline is Bungie (Destiny)
So it’s pretty well known that “Bungie” was the punchline to a dirty joke, and that’s how they chose the name. It’s of course a carefully guarded secret. But through my research I’ve been able to uncover and confirm the joke.
The thing is though… it’s been sitting in front of everyone for quite some time. All is actually revealed in the Marathon 2 source code. You might have to put two and two together, but the answer to the mystery is already out there. If you want to know before the book comes out, have a look through. :-)