Green’s Dictionary of Slang

without any money.

1833		D. Crockett Sketches and Eccentricities 60: He awoke next morning flat without a dollar.
1842		Comic Almanack June 318: There goes, Tom Gad, a twenty pounder / As flat, you are, as any flounder.
1885		Bulletin (Sydney) 28 Feb. 7/4: ‘Miss Clara,’ he finally ventured, ‘I don’t think you have much money left. This purse seems to be a little flat, he! he! he!’ ‘Flat? Well, it’s a little flat, sure enough, but I don’t mind that; it’s a big flat that I object to.’ He left her at the next corner.
1895		‘Banjo’ Paterson ‘Our New Horse’ Man from Snowy River (1902) 31: They lost their good money on Slogan, / And fell, most uncommonly flat.
1902		Ade Forty Modern Fables 9: As between the Generous Young Fellow who is Flat and the Moneyed Man who never Comes Up, it is about Six of one and Half a Dozen of the other.
1905		Duke Tritton’s Letter n.p.: A Pot & Pan driving a nice high stepping Tomato Sauce in a flash Big An’ Bulky pulled up and asked if I was looking for Dodge An’ Shirk. Being as flat as a goanna drinking at a Billabong I replied, ‘My blanky oath’.
1911		T.A. Dorgan Silk Hat Harry’s Divorce Suit 27 Nov. [synd. cartoon strip] Say Bunk slip me a couple of bucks [...] I’m as flat as a pancake.
1921		Dos Passos Three Soldiers 386: I’ve been flat fifteen days.
1932		(con. 1917–19) Dos Passos Nineteen Nineteen in USA (1966) 357: I’m flat and those goddam Scotchmen wont advance us any pay.
1932		Chelmsford Chron. 1 Dec. 3/2: I did it for food and rent. I was absolutely flat out.
1943		J. Mitchell McSorley’s Wonderful Saloon (2001) 125: I hate to bother you, pal, but I’m flat.
1951		M. Spillane Long Wait (1954) 139: She left him flat somewhere along the line.
1960		A. Seymour One Day of the Year (1977) I i: hughie: (feeling in his pocket) Sorry, I’m flat.
a.1964		‘Honky-Tonk Bud’ in D. Wepman et al. Life (1976) 55: ‘Now I ain’t flat,’ said the beat-up cat, ‘We’re traveling boosters, you know.’.
1981		J. Ellroy Brown’s Requiem 211: I only have sixty-five dollars on me [...] I’m almost flat, buddy.
1981		W.D. Myers Monster 149: I’m so flat I ain’t got enough money to buy a can of beer.
2000		(con. 1940s–60s) Décharné Straight from the Fridge Dad 60: Flat as a matzoh Broke, out of cash.
2023		(con. 1962) J. Ellroy Enchanters 182: ‘Pizza and booze would be nice, but we’re flat’.