An expression of great or utter surprise, bewilderment, or astonishment. I was so taken aback when I found out I'd won the lottery that you could have knocked me over with a feather!
Fig. I was extremely surprised.; I was so surprised that it was as if I was disoriented and could have been knocked over easily. When she told me she was going to get married, you could have knocked me over with a feather. John: Did you hear that they are going to tear down city hall and build a new one—price tag twelve million dollars? Sally: Yes, and when I heard that, you could have knocked me over with a feather.