In American usage, one is “caught flat-footed” when he is unprepared, asleep at the switch, inattentive, or surprised.
Previously, we said that the expression probably arose from the American game of football, “for it applies most pertinently to the player who, having received the ball on a pass, is caught by an opposing player before he has moved from his tracks.”
Our friend, Sir St. Vincent Troubridge, insists, “This is without doubt at all from horse-racing, quite solidly established in this country (England) from the reign of Queen Anne.”
In further correspondence he added: “In this country horse races are not started from stalls as with you, but by the horses advancing in line to a ‘gate,’ or barrier of tapes and webbing, which is raised by the starter by means of a lever when he is satisfied that the horses are in line.
It will be clear that any horse which is ‘caught flat-footed’, i.e., with all four feet on the ground, instead of dancing forward on his toes, so to speak, when the ‘gate’ rises, is at a disadvantage and will lose many lengths at the start.
Mutatis mutandis the same would apply to men at the start of a foot-race, who are normally right forward on their toes awaiting the pistol shot.”