On the back/front foot
To be on the back foot generally means to be on the defensive or to be unprepared, while to be on the front foot means the opposite. When the word ‘caught’ is added, the level of defence or unpreparedness is more definite. The figurative meaning of these phrases date from the 1930s and their literal meaning is believed to originate from sport, but it is not certain from which sport. Some sources maintain the source is cricket, while others maintain the source is boxing. Because these expressions are not as familiar in America, cricket is probably more likely, except that a batsman in cricket is just as likely to be successful on either the front or back foot. Thus the exact sporting context remains debatable. Americans generally prefer to use the expression ‘to be caught flat-footed’, which definitely does derive from baseball.