Where does the phrase “to be caught flat footed” come from and What does it mean?

Unlike “flat-footed” in the phrase, “to come out flat-footed,” one is caught flat-footed when undecided or unprepared.

It is likely that the term came 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.