It's a dude who goes on a jerk off marathon for hours at a time, and I'm sure there's a few that will do days. It can be any combination of edging or shooting one off as soon as he's reloaded. Wouldn't even necessarily have to be with porn, gooning is all about the peen. Some say that a proper goon sesh is a transcendant meditative state, where the mind is no longer present and has merged all consciousness with the dong mind.

Basically they de-evolve into bonobos.