To piss in someone's morning coffee?

Got a dialog between two officers:

"A: Sobel's a genius. I had a headmaster just like him in prep school. I know the type.

B: Lewis, Michelangelo's a genius, Beethoven's a genius.

A: You know a man in this company who wouldn't double-time Currahee with a full pack just to piss in that guy's morning coffee?"

"to piss in that guy's morning coffee?" - is it some kind of idiom?

To counter this practise Starbucks was born: now you have no idea whether anyone has pissed in your coffe or not, it just improves the flavor.