A Boston Cream Pie is the action you do while you creampie a woman. And what you do is insult her with a irish/Boston accent and then poor a small bag of coins on her.
"You like that you fookin gobshite?!?"
*poor's coins*
"Dude I gave this Irish girl a Boston CreamPie and she said it reminded her of her home!"
When you have a finger-full (usually the index finger) of sour cream and you sneak up on someone and smear it across their upper lip. Similar to the Dirty Sanchez but done with sour cream as in the movie Still Waiting.
While we were having nachos I Sour Cream Sanchez'ed my date. He was so pissed he Pirate'd me to get even!