flat broke
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.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for flat broke
Historical Examples
At last he says to me, 'You are flat-broke, and they'll all be down on you when you get back to the post.
The DeserterCharles King