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
Farlex Dictionary of Idioms.

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:
  • a stranger to (someone or something)
  • be out of (one's) league
  • be out of somebody's league
  • accompany (one) on a/(one's) journey
  • accompany on a journey
  • brief (someone) about (someone or something)
  • brief about
  • be (not) a patch on
  • be in bad with (someone)
  • (one) puts (one's) pants on one leg at a time