Where does the expression “to come out flat footed” come from and What does it mean?

There are no ands, ifs, nor buts in the statement of one who “comes out flat footed”; you know exactly what he means and where he stands.

We picture him as planted firmly on both feet, with an air of defiance, daring one to try to shake him from his opinion.

The expression originated in the United States; the earliest record of its use yet found is 1846, though “flat footed” alone, in the sense of determined, positive, is found some twenty years earlier.