The art of stealthily putting your dick in someones pocket and urinating in it.
Basically pick-pocketing but in stead of stealing, you're giving them something
Man, the bus was super crowded today, AND some bastard piss-pocketed me! (Piss-Pocketing)
Pissing in a person's pocket means to insincerely attempt to convince a person that you're doing them a favour, when you actually have only your own interests (generally either making a profit or ingratiating oneself) at heart.
i.e. You are pissing in a person's pocket, and trying to convince them that you're doing them a favour by warming their pocket. But realisticly all you're doing is soaking their pants in urine.
"That salesman was a complete dickhead. Constantly pissing in my pocket, trying to tell me how much I needed (insert product here)."