The rumored name comes from a "reliable source" close to Droid Life, but it should be made clear that the names Nexus 5X and Nexus 6P may not be final, and may simply be internal names to help differentiate them from the older Nexus 5 and 6 devices. With the Nexus 5X, it should be pretty clear that we're living in 1998 and the "X" stands for XTREME!, but we're not so sure about the "P" in Nexus 6P. It would seem redundant that it stands for "phablet". Maybe Google is going the way of Apple and Samsung and it stands for "plus".
Whatever the name means, we should be finding out soon enough as the new Nexus devices are expected to be announced at the end of this month.
So, let us know what you think the "P" stands for in the comments!
source: Droid Life