catch (someone) flat-footed
To startle someone; to come upon someone unawares and/or when they are unprepared. Usually used in the past tense. The security guard caught the thieves flat-footed. That essay question in the exam caught me completely flat-footed.
caught flat-footed
Caught unprepared, taken by surprise, as in The reporter's question caught the President flat-footed. This usage comes from one or another sport in which a player should be on his or her toes, ready to act. [c. 1900]
catch someone flat-footed
take someone by surprise or at a disadvantage. informal The opposite of flat-footed in this metaphorical sense is on your toes (see toe).
1998 Field Farming and forestry were both caught flat-footed when fashion changed.