When someone is in a particularly foul mood first thing in the morning. Many people will have a cup of coffee before anything else is taken care of. Therefore their coffee must have been pissed in to start their day off so badly.
Joe: "Hi Jack! Wonderful day isn't it?"
Jack: "Ah fuck yourself you fucking fuck!"
Joe: "Well who pissed in your coffee this morning asshole!"
To pour coffee is to plug a drain with feces. Most often said in reference places such as public restrooms and water fountains. The term originated from how the water turns brown with shit. Waffle stomping is a similar activity, but more associated with a private setting.
Person 1: Dude, I saw someone pouring coffee in the water fountain.
Person 2: Shit, now I can't hydrate.
Trying to fit in by doing something you don't normally do. Most commonly said when an individual pretends to like coffee at work to fit in with coworkers who are drinking it.
Joey: Why is Ryan acting like he knows about cars?