Shit Vicious
A euphemism to describe being suddenly overcome by the urge to defecate. The notion behind the euphemism is that one has been accosted by some rude and unpleasant character who has prevented you from performing some previously agreeded to task.
Mike: Dude, I though you were going to get us a couple of pints?
John: That was the plan until Shit Vicious came around.
John: That was the plan until Shit Vicious came around.