Proelium was chosen as a Latin word meaning 'battle'. However, it subsequently turned out that proelium had been a misspelling to start with and the Latin word was actually prolium!
When Pro moved to HHnet there had already been various shake-ups in the group's structure, including the loss of some posts made since its last backup and the departure of members. Many of the members have viewed this as a new start, and wanted this reflected officially in the name. (More on Pro's History)