knock (someone) down with a feather, to

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