[set back on one's heels] or [knock back on one's heels] {v. phr.}, {informal} To give an unpleasant surprise; upset suddenly; stop or turn back (someone's) progress.
* /Jack brags too much and it set him back on his heels when the coach told him he wasn't as good a player as he thought he was./* /Jean was doing very well in school until sickness knocked her back on her heels./
Compare: KNOCK FOR A LOOP, THROW FOR A LOSS.