knock down with a feather

knock (someone) down with a feather

To shock, confuse, or astonish someone to a point of complete bewilderment; throw someone for a loop. More often phrased as "you could have knocked me down with a feather," expressing great bewilderment or surprise. It really knocked me down with a feather to hear that my ex-wife had already remarried. I was so taken aback when I found out I'd won the lottery that you could have knocked me down with a feather!
See also: down, feather, knock

knock down with a feather

see under knock for a loop.
See also: down, feather, knock

knock (someone) down with a feather, to

To overcome with surprise. This hyperbole dates from the early nineteenth century. An early appearance in print is in William Cobbett’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).
See also: down, knock