The act of urinating, not due to a biological need or urge, but as a precaution before entering a situation in which excusing one's self to the facilities would be either inconvenient, detrimental, or a social faux pas.
Steve: "Hey, how'd the job interview go?"
Juan: "Their bathrooms were locked, so I couldn't take a safety piss beforehand. Really kinda threw off my mojo."
Donny: "Hey man, save my seat."
James: "Where are you going?"
Donny: "Taking a safety piss; this movie's three hours long!"
Craig: "I really wish I'd taken a safety piss."
Minister: "And do you, Craig, take Tina to be your lawfully wedded wife..."