I was curious too, took a little bit of digging :) "the original domain of [P]rocessing was proce55ing.net, so people used to sometimes refer to processing as

I was curious too, took a little bit of digging :) "the original domain of [P]rocessing was proce55ing.net, so people used to sometimes refer to processing as proce55ing or P5 or p5 for short. they still do sometimes. p5.js is a reference to that." from https://github.com/processing/p5.js/issues/2443 https://github.com/processing/p5.js/issues/2443