'Piss-arse' is a synonym (however, not exclusively a synonym) of 'diarrhea'. The literal meaning is 'an arse (or anus) that expels piss (urine) rather than faeces'. Breif etymology: It is believed the original usage was as an alternative word for dysentry and developed a secondary usage as either an insult or as baseless slander.
Statement: I've got piss-arse from eating raw chicken
Insult: You're a piss-ared mother shitter; I saw it you dirty scumbag!