having a poo at paul's
Leaving the place where you are to seek out a toilet, because you really badly need a shit and can't possibly have one in the place you are, given the available facilites
"Were did Ray go?"
"He is having a poo at paul's, the toilets here are rank, he'll be back in twenty minutes"
"He is having a poo at paul's, the toilets here are rank, he'll be back in twenty minutes"