verb (intr) informal
to die or be so near death that the display of one's vital signs on medical monitoring equipment shows a flat line rather than peaks and troughs
to remain at a continuous low level
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
by 1998, from the flat line on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.
Online Etymology Dictionary, © 2010 Douglas Harper