I have never heard this phrase in reference to water conservation. Anyone who has been to a cottage or country house in the middle of nowhere should be familiar with this concept. (Or at least if it was in the nether-regions of Canada.) The plumbing systems tend to be very exact with regard to how much the septic tank can hold. Flushing everytime one goes pee sends water down the pipes, taking valuable space in the septic tank. This rule prevents the tank from backflowing and drowning everyone in their refuse. This is not the way you want a weekend retreat to end.