When the performance of something
significantly degrades over time.
The names comes from the Nvidia Kepler GPU
architecture (GeForce GTX 600&700 series), which was infamous for suffering severe and steadily bigger performance losses in games after its release.
This was caused by poor optimisation in Nvidia’s drivers after the release of the Maxwell
architecture (GeForce GTX 900 series), as without shader optimisation the raw performance of Kepler cards fell off a cliff.