Green’s Dictionary of Slang

1. (also flatten out) to knock down.

1885		Bulletin (Sydney) 25 Apr. 9/1: Why the genus ‘dude’ was created is strange. Why he got through ‘knuckle-down’ and ‘rounders’ at school without being flattened out by his comrades, is odder still.
1892		J. Newman Scamping Tricks 124: Off you go or I’ll flatten you out.
1915		Van Loan ‘Sporting Doctor’ in Taking the Count 53: I never flattened him before in my life.
1923		P. MacGill Moleskin Joe 223: Well, flatten me out, if you’re not a catch!
1931		D. Runyon ‘Hold ’Em, Yale!’ in Runyon on Broadway (1954) 155: As fast as they are flattened they get up and keep belting away.
1945		J. Henderson Gunner Inglorious (1974) 172: He hauled off and flattened one with a beauty swipe.
1955		B. Schulberg On the Waterfront (1964) 69: He got himself flattened and kicked for good measure.
1963		(con. 1944) L. Glassop Rats in New Guinea 9: If Old Gutsache hadn’t stopped it I’da flattened you.
1973		E. Bunker No Beast So Fierce 121: The gorilla can flatten anybody who flinches wrong.
1982		J. Davis Dreamers 80: eli: Bastard deserved all he got. peter: Yeah, an’ I give it to him, flattened him.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 19: He’d just [...] flattened two people who didn’t get out of the way quick enough.
1996		(con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 86: ‘I’ll flatten the first bitch that tries to race you off’.
2003		Silver Ravenwolf Witches’ Night Out 110: You so much as touch her and I’ll flatten you.

2. fig. use of sense 1, to defeat.

1900		Bulletin (Sydney) 13 Jan. 14/2: Audibly observing […] that ‘the second-class people should be kept to their position,’ she was utterly flattened-out by little Mrs. Ryan’s remark in a clear voice […] – ‘We may be second-class people, Mrs. --, but thank God, we ain’t second-hand!’.
1902		Ade Forty Modern Fables 123: He gave no sign of returning to the Scratch, so she sought her own Room, leaving him all Flattened Out.
1912		Bulletin (Sydney) 21 Nov. 12/1: The Keeper of the Keys: ‘Ah! flattened out by high rents, dear meat, etc., I presume?’ / New Spook: ‘No, sir! Merely an argument with a Sydney tram.’.
1931		R.E. Howard ‘Texas Fists’ Fight Stories May 🌐 How about it, amigo? Will you mosey back up in the hills with us and flatten this big false alarm?
1976		D. Ireland Glass Canoe (1982) 14: He got on the Bacardi and that really used to flatten him. It flattened me the only times I got on it.

3. to kill, to murder.

1931

M. Harris ‘Facing the Mob’ in Gangland Stories Feb. 🌐 Bumped the sucker—flattened him—knocked him off.