Green’s Dictionary of Slang

(US) destitute, penniless.

1858

‘Philip Paxton’ Piney Woods Tavern 204: The quondam owner is said to be flat broke or flat footed and must beg, borrow, or steal for a stake. [Ibid.] 94: And then if you are flat-footed, I’d lend you a stake to start on [HDAS].