From Longman Dictionary of Contemporary Englishflat-footedˌflat-ˈfooted adjective 1 MIhaving flat feet2 informalCLUMSY moving in an awkward way SYN clumsy The defence looked flat-footed as Sutton scored easily.3 → catch somebody flat-footed
Examples from the Corpus
flat-footed• A good way to see the land without getting bow-legged doing it, or flat-footed.• The last thing they would expect was the obvious, it was a certain way to catch them flat-footed.• My smile stays really charming as I stop forward and kick, flat-footed, against the edge of the door.• Leonard was flat-footed and slow during the match.• His toes were aching so much that he went upstairs very slowly, walking flat-footed to avoid bending his tingling ankles. 