Also, stone or stony broke. Completely penniless. For example, I can't help you—I'm flat broke, or He's stone broke again. The first term dates from the mid-1800s and uses flat in the sense of “completely” or “downright.” The variant dates from the late 1800s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use flat broke in a sentence
Within six months he was flat broke and on the verge of being disbarred.Ted Williams: My Time With the Homeless Man With the Golden Voice | Mansfield Frazier | January 7, 2011 | THE DAILY BEAST
Hungerford used to pay him, but now that he is flat broke and can't help no more, he won't give him a cent.Cap'n Dan's Daughter | Joseph C. Lincoln
He was broke, flat broke, about the time I was elected and suddenly he had all the money he required.Spring Street | James H. Richardson
Yer flat broke, an' need coin, an' I'm takin' at yer own word—thet ye don't care overly much how ye git it.The Devil's Own | Randall Parrish
I let him know you needed work, but I didn't tip my hand you was flat broke.The Easiest Way | Eugene Walter and Arthur Hornblow
I'd take it to New York myself, but you see, I am flat broke.The Carpet from Bagdad | Harold MacGrath