You could have knocked me down with a feather
Definition 1 of 1
Pro Tip 1/3Figurative speech
This phrase is not literal; it means the speaker was very surprised. 
He won the lottery? You could have knocked me down with a feather!
Pro Tip 2/3Emotional impact
Use when an event or news affects you deeply and unexpectedly. 
They named the charity after me? You could have knocked me down with a feather!
Pro Tip 3/3Informal usage
The phrase is informal, best used with friends, family, or in casual conversations. 
You're retiring this young? You could have knocked me down with a feather!