XP stands for "eX Princetonian". This name was chosen because most participants (especially in the early XPs) were students of Jeffrey D. Ullman at Princeton or visiting researchers there.
The numbering comes from the idea that the XP workshops were the seed for the founding of the PODS conference. By this logic, the first PODS was XP3, the second XP4, and thus, when an XP workshop was held in the summer of 83 (after the second PODS) the "right" numbering was XP 4.5. When another XP was held in 86 it had to be numbered between 7 and 8. Since it was held later in the summer than XP 4.5, it became XP 7.52.
Acknowledgement: Information about the XP Workshops was contributed by Henry F. Korth.Last update Sun May 26 05:26:26 2013 CET by the DBLP Team — Data released under the ODC-BY 1.0 license — See also our legal information page