catch someone flat-footed
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.
See also: catch
catch someone flat-footedtake 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.