explain idiom: "rock back on heels"

[jordan89pl]

Hello,
could you please help me explain this idiom:rock back on heels.
for example: Why was the teacher rocked back on his heels?

[mmasny]

Here you go: Idiom: rock back on heels- explain it, please
It can probably be used literally too - why not?
I'm not a teacher.

[BobK]

Hello,
could you please help me explain this idiom:rock back on heels.
for example: Why was the teacher rocked back on his heels?

I've always assumed it was a boxing metaphor. An attacking boxer has his weight on his toes. When he gets hit badly, or in a surprise attack, he's 'set back on his heels'. From there it transferred to any big surprise or disappointment. It was used in the ('50s?) song Goody goody

So you met someone who set you back on your heels
Goody, goody
You met someone and now you know how it feels
Goody, goody
...

b