IDIOMS

on the back foot

(idiomatic) In a defensive posture; off-balance.

Example: 2003, Val McDermid, The distant echo, page 184:
  Lawson wasn't going to be put on the back foot by Brian Duff. "I don't know what you're on about. But I've kept my nose clean for over twenty years.
2005, Clare Naylor; Mimi Hare, The Second Assistant: A Tale from the Bottom of the Hollywood Ladder‎, page 289:
  Now I was the one on the back foot. He liked me. "Really?" "I'd like to take you out when we're back in LA"
2008, Sue Limb, Zoe & Chloe on the Prowl‎, page 112:
  She was on the back foot now. I had taken the initiative good and proper.