You could have knocked me down with a feather!
https://forum.english.best/uploads/default/original/2X/f/ff071baa4b58bbd2c858fbf8b5321f6b068a29f7.mp3
I think it means ‘I’m shocked or surprised by your action’.
Absolutely right, Natalia. I would add ‘very’ or ‘extremely’ to ‘surprised.’.