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Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs