knock (someone) down with a feather, to

knock down with a feather

see under knock for a loop.
See also: down, feather, knock
The American Heritage® Dictionary of Idioms by Christine Ammer.

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
The Dictionary of Clichés by Christine Ammer
See also:
  • knock about with (one)
  • don't knock it
  • knock
  • knock (something)
  • knock up
  • knocked up
  • knock (one) dead
  • knock dead
  • knock 'em dead
  • knock someone dead