safety poo

A poo that you take before you leave to go out to a party or on a night out, so that you won't need a shit later on.

Mike: I'm back, I was just having a safety poo, any of you guys need one? You can use my toilet..
Tom: No, I'm good, I had one before I came round.
Mike: Ben?
Ben: I tried but it just won't come out.