on his heels

[blouen]

This discovery of the beginning of the universe and its precisely fine-tuned laws has set materialists on their heels.

What does it mean to say, "something set someone on his heels."

[riverkid]

This discovery of the beginning of the universe and its precisely fine-tuned laws has set materialists on their heels.

What does it mean to say, "something set someone on his heels."

It means taken aback, so surprised or shocked that you tend to recoil, shifting your weight to your heels.