Green’s Dictionary of Slang

1. of a person, to die; also of a faculty.

1982		W. Safire What’s The Good Word? 152: ‘To flatline’ is to expire, a verb taken from the lack of activity on the scope measuring vital signs.
1988		Maledicta IX 198: Patients croak [or] flatline.
1994		J. Wambaugh Finnegan’s Week 291: He’ll flat-line before he’s much older.
1999		Dr Dre ‘Some L.A. Shit’ 🎵 Anybody hatin on us can suck a dick / If I catch you touchin mine you catch a flatline, dead on the floor.
2000		J. Ellroy ‘I’ve Got the Goods’ in Destination: Morgue! (2004) 176: Their collective circulation fizzled. They flatlined into the ’60s.
2004		L. Redhead Peepshow [ebook] He flatlined in the ambulace and never recovered.
2005		J.W. Hill Natural Law 337: ‘He’s gone flatline.’ She heard the horrible whine of the monitor.
2011		F. Bill ‘Officer Down’ in Crimes in Southern Indiana [ebook] [H]is body snuggled with cold, his hearing flatlined.
2012		(con. 1980s) I. Welsh Skagboys 375: The cunt must be a baw hair away fae flatlining, the wey he’s been battering the shit intae hissel.
2020		(con. 1991-94) W. Boyle City of Margins 204: She opens her eyes to the sound of Don flatlining.

2. of an inanimate object, to fail, to collapse, fade out.

2000		Burkhart & Hunt Airstream: The Hist. of the Land Yacht 61: Design innovations for trailers flatlined, waiting for cues from the aircraft industry.
2004		J. Ellroy ‘Hot-Prowl Rape-O’ in Destination: Morgue! (2004) 267: Their careers will flatline, their retirement job prospects will tank.
2015		L. McInerney Glorious Heresies 123e: The joke flatlines.
2023		(con. 1962) J. Ellroy Enchanters 263: The lobster salad wilted. Our easy talk flatlined.

3. as vtr., to demolish, to strike dead.

2023

(con. 1962) J. Ellroy Enchanters 53: The lovefest lasts three hours. A big row with George Cukor flatlines it.