the.com/meltdown

means A complete collapse of control — emotional, mental, or mechanical — where a system that was holding steady suddenly can't anymore.

from Born in nuclear engineering in the mid-20th century, describing the literal nightmare scenario where a reactor's core overheats, melts through its containment, and loses all structure. By the 1970s–80s the word escaped the power plant and went psychological, applied to anyone whose composure overheated past the point of containment — the metaphor sticking because it's exact: something solid, meant to hold, turning molten under pressure.