Popular euphemism for 'I'm going to go do Doctor Poo.'
Other meanings of Doctor Poo are as follows,
Dropping the kids off at the pool
Shaking hands with Tom Titt
Choking a dark demon
Doing Ertha Kitt
My arse was just sick
Talking to god on the porcelain telephone
and the best one...
emailing your boss's letterbox with a dirty attachment.