The term, "Piss Sack" refers to an artificial bladder, affectionatley known medically as a Colostomy Bag, which can be worn strapped to an inner thigh, or worn outside the body for effect.
The term "Piss Sacker" refers to a particular wearer of this device - one who has nothing wrong with their urinary control functionality, but rather a person who is not keen to allow the disruptiveness of standing up and walking to a near by toilet interfere with their every day routine. Particularly beneficial when binge drinking.
Person 1 - Fuck me mate, we've only had three pints and I'm already on my second piss....
Piss Sacker - You have an absurdly weak bladder (he he he)!
1 Hour later.......
Person 1 - Look this isnt funny anymore - This is our 17th pint and you havn't had a piss yet. Whats going on? Are you some kind of piss sacker?
Person 2 - Yes.