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!
knock down with a feather
see under knock for a loop.
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).