set back on one's heels?

[Kenneth_Wai]

set back on one's heels
Teacher, may I ask what is the meaning of this idom?

Set you back means "strike you heavily and make you feel not comfortable" but I could figure out the whole phrase.

Please advise, thank you!

[mara_ce]

[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.