Green’s Dictionary of Slang

totally impoverished.

1865		Lincs. Chron. 28 Jan. 2/5: Haven’t got any money [...] Got none, I’m flat broke.
1872		Schele De Vere Americanisms 602: The same word enters into the phrase flat broke, meaning the same as ‘dead broke,’ from the idea of being so broken as to lie flat on the ground.
1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 274: When we see the raft was gone, and we flat broke, there wasn’t anything for it but to try the Royal Nonesuch another shake.
1885		Aberdeen Eve. Exp. 4 Aug. n.p.: Mr Fresh (the silly ‘bloke’), / Who does his cash in Wall Street soak, / And goes home later, flat-dead broke — That’s a lamb.
1895		Edinburgh Eve. News 4 July 4/5: One year he is a millionaire, and the next year he is flat broke.
1902		J.D. Corrothers Black Cat Club 38: Played de races—got broke flat.
1914		R. Lardner You Know Me Al (1984) 113: I am flat broke Al and all I am asking you is to send me enough money to pay my fair to Bedford.
1922		P.A. Rollins Cowboy 87: The Westerner, having started life when financially ‘flat broke’.
1939		K. Tennant Foveaux 279: Here am I—practically flat broke. Don’t know if I can scrape together enough to pay my rent next week.
1949		H. Miller Sexus (1969) 47: That would leave me flat broke.
1957		Kerouac letter 24 June in Charters II (1999) 46: I wouldnt be surprised if Viking Press [...] put off the publication of my book and leave me flat bum broke again.
1965		J. Thompson Texas by the Tail (1994) 158: ‘Need any scratch [...]?’ ‘No, I’m not completely flat.’.
1971		B. Moyers Listening to America 103: We’re flat broke.
1985		T. Wolff ‘The Rich Brother’ in Back in the World 201: His car had been repossessed [...] and he was flat broke.
1997		Simon & Burns Corner (1998) 60: We’ve got to begin to think as Gary McCullough thinks when he’s flat broke and sick with desire.
2000		W. Shaw Westsiders 159: As usual, he’s flat broke.