When you are wearing a jacket zipped up all the way to the neck, with hands in pockets, waiting to kick the shit out of someone, as seen in the movie Green Street Hooligans.
John: Why the hell is that guy standing like that? Does he not know he's about to get the shit kicked out of him?
Steve: No you tosser! He's Greenstreeting. He'll beat the fuck out of that mick bastard.
A person's metaphoric wang, the size of which being related to his apparent devotion to the environment via principles, purchases, lame t-shirts and bumper stickers, etc. "Green" comes from the color commonly associated with environmentalism, and "peen" is short for penis. Duh.
While Ryan greatly enlarged his greenpeen by purchasing a Prius, Sean's Highlander Hybrid provided a decent enlargement while also allowing him to keep his dignity.
As is a reference to having sex on a lawn and the resulting green grass-stained clothing, greensleeves may also be used in a broader context to generally mean having sex in public.
The risk of being watched while going greensleeves during a music festival could be a Porn theme/fantasy.