Use 'aback' when something happens that is so surprising it makes someone temporarily confused or bewildered. The sudden resignation of the manager took everyone aback.
Pro Tip 2/3
Emotional Impact
Conveys a strong emotional response, usually related to surprise or shock, not just a casual reaction. I was taken aback by the sheer number of people at the rally.
Pro Tip 3/3
Negative Surprise
Often implies a negative connotation, suggesting the surprise is not pleasant or welcome. She was taken aback when she found out she was being laid off.