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from the flat line displayed on a heart monitor when the heart stopped functioning
slang; also flatline n.
by 1998, from the flat line on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.
Dead: The defendant's face was flatlineverb
To die: State government is in a coma. We're going to flat-line at any moment
[1980s+; fr the flat line showing no waves, hence no heartbeat, on a heart-monitoring machine]