Green’s Dictionary of Slang

1. insipid, maladroit.

1929		(con. WWI) H. Odum Wings on My Feet 130: Boys mighty hard on Sambo, callin’ him dam’, flat-footed, crippled Tiger.
1938		J. Curtis They Drive by Night 248: You bone-headed, flat-footed, numbskull.

2. unprepared, caught unawares; thus catch flat-footed v., to catch unawares.

1912		Amer. Mag. Jun. p.202/2 in AS XXVI:1 (1951) 31/1: Flat-footed – Unprepared, caught napping.
1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 330: to be caught flat-footed—to be caught off one’s guard.
1944		R.L. Bellem ‘Focus on Death’ Hollywood Detective Jan. 🌐 He came back within less than ten minutes; caught me flat-footed.
1953		F. Paley Rumble on the Docks (1955) 84: He don’t wanna git caught flatfooted in a rumble.
1969		‘Iceberg Slim’ Pimp 108: Here I’d had her a week and I was flat-footed.
1997		E. Little Another Day in Paradise 37: Have a good time, but never let yourself get caught flat-footed.