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Scram is an emergency shutdown of a nuclear reactor and is sometimes used as a verb meaning "to shut down." The term has been extended to refer to shutdowns of other complex operations, such as server farms and even large model railroads. The button may have also played on the word's more conventional meaning "to go away quickly," which was originated colloquially in the US in 1920s.
Folk etymology dictates that the term comes from the acronym for safety control rod axe man and was coined by Enrico Fermi when the world's first nuclear reactor was built. But the axe-man story has been debunked by historian Tom Wellock. However, the use of the word to mean "rapid shutdown" was indeed originated in nuclear physics.