Catholics (or any form of Christianity, basically) who only goes to Church on days when they give out free stuff; namely Ash Wednesday and Palm Sunday.
Bob: Do you ever go to Church?
Steve: Only on Ash Wednesday and Palm Sunday, when they give out free stuff.
Bob: Dude, you are such an A and P Catholic!
P and W stands for Piss and Walk. Commonly, when pissing in public you do not wish to draw suspicion by facing a wall for a minute. Or get left behind by the group. So instead, you piss while walking. Commonly needed in the city, especially after drinking.