A sudden, complete emotional breakdown or loss of control, or a dangerous failure in a nuclear reactor where the core becomes too hot.
Compound word from 'melt' (to turn from solid to liquid by heat) and 'down' (direction/collapse). Originally used for nuclear disasters in the 1960s-70s, then borrowed as a metaphor for emotional collapse.
The word 'meltdown' came straight from nuclear science jargon—a nuclear core melting would be a total catastrophe—and psychologists borrowed it to describe emotional breakdowns because they're equally destructive! It's a perfect example of how technical language becomes everyday slang when people need words for intense experiences.
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