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.
Examples from the Web for flat broke
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 Deserter