A water saving strategy usually used by third world countries, but now is being used in rural country Australia i.e. Bendigo etc to help save water.
It's when you re-use the water from washing dishes, laundry and bathing. It's dangerous to drink or give to animals and is not suppose to be used to water plants or trees that produce fruits or foods for human consumption i.e. apple trees, tomato plants etc
"Did you just piss in the shower?"
"Ewww now our grey water is gonna be yellow water!"