1
general::
Phrase(s): flat broke [and] flat busted
Fig. having no money at all. • Sorry, I’m flat broke. Not a cent on me. • You may be flat broke, but you will find a way to pay your electricity bill or you will live in the dark. • Mary was flat busted, and it was two more weeks before she was due to get paid.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms