Negative returns
Etymology
An extension of the economic language of "returns" (from Old French "retorner," to turn back), which describes the output gained per unit of input. “Negative” marks the stage beyond diminishing, where the curve turns downward.
noun phrase · /NEG-uh-tiv ri-TURNZ/
The point at which adding more of an input actually reduces total output, rather than merely adding less than before.
What it names
It marks the threshold past which effort becomes actively counterproductive — not just inefficient, but harmful to the result.
Example
A third all-nighter brought negative returns: he was so exhausted the report got worse, not better.
Related idea
The stage beyond diminishing returns: with diminishing returns each extra unit still helps, only less; with negative returns each extra unit subtracts from the total, so the rational move is to do less.