Flat Broke

To be “flat broke” is to have absolutely no money. The word “flat” is added, perhaps, to emphasize that there is no bulge in any pants pocket to show even a single coin. “Flat broke” has been cited in print since at least 1840.
 
   
(Oxford English Dictionary)
broke, adj.
Freq. with qualifying word, as broke clean, broke dead, flat broke, stone-broke, stony broke.
1842 Spirit of Times 2 Apr. 58/1   Barrett, poor fellow, is dead broke.
1842 Spirit of Times 21 May 138/1   Every friend of Old Whitenose would have been flat broke!
1843 Spirit of Times 14 Jan. 544/3,  I was clean broke in less than four hours.
1846 Spirit of Times 25 Apr. 101/2,  I unfortunately am short of funds, flat broke, busted, collapsed.
 
23 September 1840, Ohio Statesman (OH), “Bank Swindlers in Mississippi,” pg. 4:
You must not presume from the above, that I am absolutely “flat broke,” and troubled with the blue devils.
 
18 November 1842, New Orleans (LA) Picayune, pg. 2, col. 5:
“I’m flat broke!” as the flatboat said when striking on a snag.
 
14 February 1844, Lorain Republican (Elyria, OH), “A Texian Hero,” pg. 1, col. 4:
About the time the Texas excitement ran so high in the United States, Jim Wills was in Pittsburgh, in that situation so common to play actors, viz: “flat broke.”
   
Google Books
The Mysteries and Miseries of New York:
A story of real life
By Ned Buntline
New York, NY: W.F. Burgess
1849
Pg. 36:
No; I am broke, flat broke! I haven’t a dollar left!”