Psunj was called Pisunus in antiquity. There are several suggested etymologies for that. One is that it comes from the Indo-European root *pekj (cattle). The other is that it comes from the Indo-European roots *peiH (big, in the sense mountain) and *sen (old). One very similar to that is that it comes from *peiH and *sewn (black). The other is that it's related to the Latin word pinus, meaning pine tree or resin, itself without a widely accepted etymology.