Definition of you could have knocked me (or her, him, etc.) down with a feather in US English:

you could have knocked me (or her, him, etc.) down with a feather

phrase

informal
  • Used to express great surprise.

    • ‘But when I read it, you could have knocked me down with a feather.’
    • ‘I felt sure it was something physical like a virus, so you could have knocked me down with a feather when he diagnosed depression.’