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"de-magnetize," originally especially of ships as a defense against magnetic mines, 1940, from German scientist Carl Friedrich Gauss (1777-1855), a pioneer in the study of magnetics.
To neutralize or rebalance the magnetic field of a magnetized object, such as a computer monitor or the read/write head of a disk drive or tape recorder.
To erase information from a magnetic disk, tape, or other magnetic storage device.