to rip the piss is to point fun at someone, slag them off, take the mickey or generally have a laugh at the expense of someone else.

When you have held in a piss for too long and it spurts out at a high velocity due to the pressure that has built. It can also simply refer to the need to go pee.

To joke or lie about someting in a humorus manner.

When someone gets their words mixed up and says "Rip the Piss" instead of "Rip the shit" or "Take the piss".