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)."