Short for 'Guide to the Project Management Body of Knowledge'—a kind of encyclopedia for project managers. Or, a comedy of manners—depends what you like.
PMBOK is a publication put out by the Project Management Institute every so many years. The 'Guide to' part was added as an attempt at humility—PMI decided that their book wasn't going to be able to contain quite the entire body of knowledge of the entire PM universe after all.
I think they're just not playing to win...