The isolated, tucked away bathroom that you scout out when at someone else's house. It is the optimal place to dump your load when you're trying to avoid the embarrassment of the effects of a digestive explosion (smells, stains, etc.).
Bro 1: "Dude I need to take a ginormous shit but I don't want his hot sister to find out."
Bro 2: "Chillax, there's a Designated Shit Bathroom in the guest house."