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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.