In the vein of the term "red pilled", in which a person becomes more of a furry based on the content they consume. Generally references anthro furry characters which might've inspired someone to become a furry, either subconsciously or consciously.
When someone is piss pilled, it means that they are generally an angry person who is always pissed off, and will likely be miserable their whole lives due to their constant irritability. Dubbed piss pill because of the whole red pill/blue pill whatever pill concept of the matrix, but unlike the other pills, the piss pill makes it's users live a live of "pissful ignorance," or live a life where everything sucks and they're always pissed off.
Person 1: "Bro what's that guy's problem? Why is he such a jerk?"
Person 2: "Ahdon't mind Ryan. He's always like that. He's Piss pilled."