Green’s Dictionary of Slang

downright, positive, undeviating, straightforward, thus adv. flat-footedly.

1828		A.N. Royall Southern Tour II 114: He was one of your right down flat-footed ox-drivers .
a.1848		in Bartlett Dict. Americanisms 145: Col. M— attempted to define his position, but being unable, exclaimed: I’m an independent, flat-footed man, and am neither for nor against the mill-dam.—Tennessee Newspaper.
1858		Harper’s Mag. Sept. 563: His herculean frame, and bold, flat-footed way of saying things, had impressed his neighbours, and he held the rod in terrorism over them .
1890		Nat. Police Gaz. (NY) 18 Jan. 2/2: Ibsen doesn’t veneer like Dumas and Sardou. He comes out flat-footed. Disease, sexual immorality, marital infidelity are paraded.
1893		Sheffield Dly Teleg. 10 Nov. 7/6: [headline] A Big Flatfooted Fib.
1898		H. Macfall Wooings of Jezebel Pettyfer 319: Yo’ blamed flat-footed ijiot!
1908		N.Y. Eve. Telegram in Fleming Unforgettable Season (1981) 154: Frank Chance [...] is flatfooted against the ‘spit ball’.
1944		E. Wilson 16 Mar. [synd. col.] [A]s close as I ever get to a downright, flatfooted, out-with-it-boys opinion.
1945		E. Wilson 9 Feb. [synd. col.] He felt strongly, if not flat-footedly, that one should go to bed early.
1969		‘Iceberg Slim’ Pimp 31: I am the best flat-footed hustler in town.