for German Größter Anzunehmender Unfall
, "biggest assumable accident
". Originally, this was techspeak
for any worst case
scenario in an industrial facility. Ideally, the on-site firefighters
et al would have some way of dealing with it.
The expression came into general usage after it was (aptly) used to describe the Chernobyl disaster, and since then, it's generally used only to mean accidents involving nuclear power plants, and often the even more emphatic form Super-GAU is used.