Made me delve a bit, the logical conclusion i came to:

it seems to be evolved from

https://en.wikipedia.org/wiki/PSX_(digital_video_recorder)

to https://m.emuparadise.me/Sony_Playstation_Emulators/Windows/PCSX/18

to

https://en.wikipedia.org/wiki/PCSX-Reloaded

and https://en.wikipedia.org/wiki/PCSX2

There is no mention on their page about it though. https://rpcs3.net/about

So an initial word play on PC and PS merged into PCS later added reloaded?