Where does the expression "you could have knocked me over with a feather" come from? My students had never heard it when I used it in class the other day.
knock (someone) down with a feather: to overcome with surprise. This hyperbole dates from the early nineteenth century. An early appearance in print is in William Cobbettt's Rural Rides (1821): "You might have knocked me down with a feather." Today it is more often used with the conditional could (instead of might)
According to the Early American Proverbs and Proverbial Phrases, it dates the idiom back to 1796.
To knock one down with a feather
1796 Cobbett Porcupine 4.131: as the old women say, you might have knocked me down with a feather. Barbour 63(7); Oxford 433;
From the Internet Archive: William Cobbet's Rural Rides
I came to a group of shabby houses upon a hill. While the boy was watering his horses, I asked the ostler the name of the place; and, as the old women say, “you might have knocked me down with a feather,” when he said, “Great Bedwin.” The whole of the houses are not intrinsically worth a thousand pounds. There stood a thing out in the middle of the place, about 25 feet long and 15 wide, being a room stuck up on unhewed stone pillars about 10 feet high.
And many thanks to Hugo who found the relevant 1796 excerpt in Porcupine's Works; containing various Writings and Selections, (...) in Twelve Volumes (May 1801)
A variant found in Samuel Richardson's Pamela; or, Virtue Rewarded (1740):
"I was so confounded at these words, you might have beat me down with a feather."