set back on one's heels
Surprise, shock, or disconcert, as in The news of their divorce set us back on our heels. This idiom, with its graphic image of someone being pushed back, dates from the first half of the 1900s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.