From the Greek "pirra" meaning fire or fiery. In the context of PKD's Exegesis it most likely refers to the Holy Spirit or plasmate through which the Pentecost is achieved. See St. Elmo, Asklepios, Siddhartha Gotama, Jesus et. al.
In Greek mythology, Pyrrha was the wife of Deucalion and the daughter of Pandora, and was named for her fiery red hair.