Someone would be better off not doing something or acting a certain way because it is or may become destructive, counterproductive, futile, or undesirable. Mark keeps antagonizing the boss with little snarky quips—he just doesn't know when to quit! My wife keeps spending our money on clothes she doesn't even wear. She doesn't know when to quit! That's the third time you've berated me for not taking out the garbage. You just don't know when to quit, do you?