Today's OED antedating: "flat-footed"

Flat-footed, figurative, colloquial, unprepared (a., sec 2 (b), first use cited 1912): Book titled The Bride and the Pennant, by Frank L. Chance, 1910, ch v, p 46: "When you end your motion and deliver the ball you are down flat-footed, and you recover slowly."