An excuse used when you get caught doing something.
RPI student hears something in his apartment when all his roommates are away. He come out of his room to find some guy robbing him. After shouting "what the fuck are you doing? Get the fuck out of my apartment!" the robber says, "Julio owes me money".
When you're screwing a girl who's got a boyfriend and he walks in.
the boyfriend: "WHAT THE FUCK?!?!"
you (calmly): "Julio owes me money"