So the thing that detergent removes. That has to be tergent, correct? And from this we can conclude that the thing that gets clothes dirty again is retergent. It’s simple logic.
(And so I fulfill last week’s promise. Thank you and please check back next week when I start to make a joke about how etymologically ‘conceal’ must mean that you’re doing something ‘with seal’ except then I realize that’s probably exactly what it does mean and I get all pouty.)