What doesn't come out in the wash comes out in the rinse.

[Deese]

I vaguely understand this saying. "What doesn't come out in the wash" means what isn't made clear I assume during the telling, showing, doing etc. If not it "comes out in the rinse." Does this mean further telling, showing or doing? What is the origin of this saying.

[Grunty]

Before we had washing machines, clothes were washed by hand.  Then the soap was removed by rinsing them in clean water.  The phrase is saying that if questioning is not productive, the truth will come out later.  People who lie need to tell more and more lies to maintain the story, and it becomes difficult to keep up the pretence.