demon core

Meanings

1. A spherical subcritical mass of plutonium manufactured during World War II by the United States as a fissile core for an early atomic bomb.

1. A essential instance or part of something, especially something malevolent.
2. Something that is especially dangerous and should not be tampered with, or otherwise reminiscent of the demon core.

Etymology

So called because of its involvement in two fatal criticality accidents at the Los Alamos Laboratory in 1945 and 1946. In the late 2010s, the circumstances of the second accident were popularized widely by science history videos and internet memes.