Größter Anzunehmender Unfall, (trans: Maximum Credible Accident) is a German term used for nuclear accidents at approximately International Nuclear Event Scale (INES) - Level 4.
Nuclear holocaust or Chernobyl would be a Super-GAU. Colloquially equivalent to
WCS (worst case scenario), or
when shit hits the fan.
When, whilst you are watching a
mushroom cloud rise on the
horizon that ugly girl from
last weekend's party calls to tell you that you got her pregnant and that she probably gave you herpes, you know you are experiencing a Super-GAU.