Take the piss expression from Suits

Hi. I've been watching the first episode of "Suits". There's a guy selling pot for a living and he's kind of in the middle of another deal. He gets a call from his friend who agree to deliver the briefcase for 25 000 dollars. When off the phone, he says to his associates: *"Yeah. He's in. (after a while) I'm going to go take the ***."* Then he goes away with the briefcase.
I've looked it up and I found out it can mean a couple of things, but I don't really think anyone matches this particular case. Or probably I just don't get what he meant. Can anyone explain me what's been on his mind?